Evolution Of Cataclysmic Variables Research Articles

Multiwavelength study of RX J2015.6+3711: a magnetic cataclysmic variable with a 2-h spin period

The X-ray source RX J2015.6+3711 was discovered by ROSAT in 1996 and recently proposed to be a cataclysmic variable (CV). Here we report on an XMM-Newton observation of RX J2015.6+3711 performed in 2014, where we detected a coherent X-ray modulation at a period of 7196+/-11 s, and discovered other significant (>6sigma) small-amplitude periodicities which we interpret as the CV spin period and the sidebands of a possible ~12 hr periodicity, respectively. The 0.3-10 keV spectrum can be described by a power law (Gamma = 1.15+/-0.04) with a complex absorption pattern, a broad emission feature at 6.60+/-0.01 keV, and an unabsorbed flux of (3.16+/-0.05)x10^{-12} erg/s/cm^2. We observed a significant spectral variability along the spin phase, which can be ascribed mainly to changes in the density of a partial absorber and the power law normalization. Archival X-ray observations carried out by the Chandra satellite, and two simultaneous X-ray and UV/optical pointings with Swift, revealed a gradual fading of the source in the soft X-rays over the last 13 years, and a rather stable X-ray-to-optical flux ratio (F_X/F_V ~1.4-1.7). Based on all these properties, we identify this source with a magnetic CV, most probably of the intermediate polar type. The 2 hr spin period makes RX J2015.6+3711 the second slowest rotator of the class, after RX J0524+4244 ("Paloma", P_spin~2.3 hr). Although we cannot unambiguously establish the true orbital period with these observations, RX J2015.6+3711 appears to be a key system in the evolution of magnetic CVs.

LONG-TERM DECREASE AND CYCLIC VARIATION IN THE ORBITAL PERIOD OF THE ECLIPSING DWARF NOVA V2051 OPH

V2051 Oph is a deeply eclipsing dwarf nova with an orbital period below the period gap of cataclysmic variables (CVs). It has been photometrically monitored since 2008 June and 24 mid-eclipse times of the white dwarf have been obtained. The changes in the orbital period are investigated using all of the available mid-eclipse times. A continuous period decrease with a rate of was discovered to be superimposed on a periodic variation with a small amplitude of 0000329 and a period of 21.64 years. The standard theory predicted that the evolution of CVs below the period gap is driven by gravitational radiation. However, angular momentum loss (AML) via gravitational radiation is insufficient to explain this decrease, and additional AML via magnetic braking that is about five times the gravitational radiation rate is required. This is consistent with the theoretical requirement indicating that magnetic braking of the fully convective star is not completely stopped. The cyclic oscillation was interpreted as the variation of the arriving eclipse time via the presence of a third body because the required energy for the Applegate mechanism is much larger than that radiated from the secondary in 10 years. Its mass is derived as Jupiter mass. For orbital inclinations it would be a planetary object. The giant circumbinary planet is orbiting around V2051 Oph at an orbital separation of about 9.0 astronomical units (AU) in an eccentric orbit (e' = 0.37). These conclusions support the ideas that some planets could survive stellar late evolution and that dwarf novae are also planetary hosting stars.

Three in one go: consequential angular momentum loss can solve major problems of CV evolution

Abstract The average white dwarf (WD) masses in cataclysmic variables (CVs) have been measured to significantly exceed those of single WDs, which is the opposite of what is theoretically expected. We present the results of binary population synthesis models taking into account consequential angular momentum loss (CAML) that is assumed to increase with decreasing WD mass. This approach cannot only solve the WD mass problem but also brings in agreement theoretical predictions and observations of the orbital period distribution and the space density of CVs. We speculate that frictional angular momentum loss following nova eruptions might cause such CAML and could be the missing ingredient of CV evolution.

White dwarf masses in cataclysmic variables

The white dwarf (WD) mass distribution of cataclysmic variables (CVs) has recently been found to dramatically disagree with the predictions of the standard CV formation model. The high mean WD mass among CVs is not imprinted in the currently observed sample of CV progenitors and cannot be attributed to selection effects. Two possibilities have been put forward: either the WD grows in mass during CV evolution, or in a significant fraction of cases, CV formation is preceded by a (short) phase of thermal time-scale mass transfer (TTMT) in which the WD gains a sufficient amount of mass. We investigate if either of these two scenarios can bring theoretical predictions and observations into agreement. We employed binary population synthesis models to simulate the present intrinsic CV population. We incorporated aspects specific to CV evolution such as an appropriate mass-radius relation of the donor star and a more detailed prescription for the critical mass ratio for dynamically unstable mass transfer. We also implemented a previously suggested wind from the surface of the WD during TTMT and tested the idea of WD mass growth during the CV phase by arbitrarily changing the accretion efficiency. We compare the model predictions with the characteristics of CVs derived from observed samples. We find that mass growth of the WDs in CVs fails to reproduce the observed WD mass distribution. In the case of TTMT, we are able to produce a large number of massive WDs if we assume significant mass loss from the surface of the WD during the TTMT phase. However, the model still produces too many CVs with helium WDs. Moreover, the donor stars are evolved in many of these post-TTMT CVs, which contradicts the observations. We conclude that in our current framework of CV evolution neither TTMT nor WD mass growth can fully explain either the observed WD mass or the period distribution in CVs.

Evolution and Outbursts of Cataclysmic Variables

Mass transfer and accretion are very important to understand the evolution and observational properties of cataclysmic variables (CVs). Due to the lack of an accretion disk, eclipsing profiles of polars are the best source to study the character of mass transfer in CVs. By analyzing long-term photometric variations in the eclipsing polar HU Aqr, the property of mass transfer and accretion are investigated. The correlation between the brightness state change and the variation of the ingress profile suggests that both the accretion hot spot and the accretion stream are produced instantaneously. The observations clearly show that it is the variation of mass transfer causing the brightness state changes that is a direct evidence of variable mass transfer in a CV. It is shown that it is the local dark-spot activity near the L1 point to cause the change of the mass transfer rather than the activity cycles of the cool secondary star. Our results suggest that the evolution of CVs is more complex than that predicted by the standard model and we should consider the effect of variable mass accretion in nova and dwarf nova outbursts.

New Method of Estimating Binary's Mass Ratios by Using Superhumps

Abstract We propose a new dynamical method of estimating binary's mass ratios by using the period of superhumps in SU UMa-type dwarf novae during the growing stage (the stage A superhumps). This method is based on the working hypothesis that the period of superhumps in the growing stage is determined by the dynamical precession rate at the 3W1 resonance radius, and is suggested in our new interpretation of the superhump period evolution during a superoutburst (2013, PASJ, 65, 95). By comparing objects having known mass ratios, we show that our method can provide sufficiently accurate mass ratios comparable to those obtained by eclipse observations in quiescence. One of the advantages of this method is that it requires neither an eclipse nor any experimental calibration. It is particularly suitable for exploring the low mass-ratio end of the evolution of cataclysmic variables, where the secondary is not detectable by conventional methods. Our analysis suggests that previous determinations of the mass ratio by using superhump periods during a superoutburst were systematically underestimated for low mass-ratio systems, and we provided a new calibration. It reveals that most WZSge-type dwarf novae have either secondaries close to the border of the lower main-sequence or brown dwarfs, and most of the objects have not yet reached the evolutionary stage of period bouncers. Our results are not in contradiction with an assumption that an observed minimum period ($\sim 77$ min) of ordinary hydrogen-rich cataclysmic variables is indeed the minimum period. We highlight how important the early observation of stage A superhumps is, and propose an effective future strategy of observation.

UPDATED PHOTOMETRY AND ORBITAL PERIOD ANALYSIS FOR THE POLAR AM HERCULIS ON THE UPPER EDGE OF THE PERIOD GAP

Twenty-one new optical light curves, including five curves obtained in 2009 and sixteen curves detected from the AAVSO International Database spanning from 1977 to 2011, demonstrate 16 new primary minimum light times in the high state. Furthermore, seven newly found low-state transient events from 2006 to 2009 were discovered, consisting of five Gaussian-shaped events and two events with an exponential form with decay timescales of $<$0.005 days; these timescales are one order of magnitude shorter than those of previous X-ray flare events. In the state transition, two special events were detected: a "disrupted event" with an amplitude of $\sim$ 2 mag and a duration of $\sim$ 72 minutes and continuing R-band twin events larger than all known R-band flares detected in M-type red dwarfs. All 45 available high-state data points spanning over 35 yr were used to construct an updated O-C diagram of \objectname{AM Herculis}, which clearly shows a significant sine-like variation with a period of 12-15 yr and an amplitude of 6-9 minutes. Using the inspected physical parameters of the donor star, the secular variation in the O-C diagram cannot be interpreted by any decided angular momentum loss mechanism, but can satisfy the condition $\tau_{\dot{\rm M}_{2}}\simeq\tau_{\rm KH}>>\tau_{\dot{\rm R}_{\rm 2}}$, which is required by numerical calculations of the secular evolution of cataclysmic variables. In order to explain the prominent periodic modulation, three plausible mechanisms - spot motion, the light travel-time effect, and magnetic active cycles - are discussed in detail.

BK Lyncis: the oldest old nova and a Bellwether for cataclysmic variable evolution

We summarize the results of a 20-yr campaign to study the light curves of BK Lyn, a nova-like star strangely located below the 2 to 3 h orbital-period gap in the family of cataclysmic variables (CVs). Two apparent superhumps dominate the nightly light curves, with periods 4.6 per cent longer, and 3.0 per cent shorter, than the orbital period. The first appears to be associated with the star's brighter states (V ∼ 14), while the second appears to be present throughout and becomes very dominant in the low state (V ∼ 15.7). It is plausible that these arise, respectively, from a prograde apsidal precession and a retrograde nodal precession of the star's accretion disc. Starting in the year 2005, the star's light curve became indistinguishable from that of a dwarf nova – in particular, that of the ER UMa subclass. No such clear transition has ever been observed in a CV before. Reviewing all the star's oddities, we speculate: (a) BK Lyn is the remnant of the probable nova on 101 December 30, and (b) it has been fading ever since, but it has taken ∼2000 yr for the accretion rate to drop sufficiently to permit dwarf-nova eruptions. If such behaviour is common, it can explain other puzzles of CV evolution. One: why the ER UMa class even exists (because all members can be remnants of recent novae). Two: why ER UMa stars and short-period nova-likes are rare (because their lifetimes, which are essentially cooling times, are short). Three: why short-period novae all decline to luminosity states far above their true quiescence (because they are just getting started in their post-nova cooling). Four: why the orbital periods, accretion rates and white dwarf temperatures of short-period CVs are somewhat too large to arise purely from the effects of gravitational radiation (because the unexpectedly long interval of enhanced post-nova brightness boosts the mean mass-transfer rate). And maybe even five: why very old, post-period-bounce CVs are hard to find (because the higher mass-loss rates have ‘burned them out’). These are substantial rewards in return for one investment of hypothesis: that the second parameter in CV evolution, besides orbital period, is time since the last classical-nova eruption.

The Evolution of Cataclysmic Variables

AbstractUsing Eggletons code the evolution of cataclysmic variables (CVs) is investigated. CVs might suffer the loss of mass and angular momentum during their evolution, we present the models of CVs with mass loss and angular momentum loss (AML) due to gravitation wave radiation (GR) and/or magnetic braking (MB). It is found that the loss of mass and angular momentum has significant influence on the evolution of CVs, and that the change of the star structure or their atmosphere properties is a possible mechanism which underlies a sudden change in the rate of AML owing to MB.

ANGULAR MOMENTUM LOSS MECHANISMS IN CATACLYSMIC VARIABLES BELOW THE PERIOD GAP

Mass transfer in cataclysmic variables (CVs) is usually considered to be caused by angular momentum loss (AML) driven by magnetic braking and gravitational radiation (GR) above the period gap, and solely by GR below the period gap. The best-fit revised model of CV evolution recently by \citet{kbp11}, however, indicates that AML rate below the period gap is $2.47(\pm 0.22)$ times the GR rate, suggesting the existence of some other AML mechanisms. We consider several kinds of consequential AML mechanisms often invoked in the literature: isotropic wind from the accreting white dwarfs, outflows from the Langrangian points, and the formation of a circumbinary disk. We found that neither isotropic wind from the white dwarf nor outflow from the $L_1$ point can explain the extra AML rate, while ouflow from the $L_2$ point or a circumbinary disk can effectively extract the angular momentum provided that $\sim (15-45)%$ of the transferred mass is lost from the binary. A more promising mechanism is a circumbinary disk exerting gravitational torque on the binary. In this case the mass loss fraction can be as low as $\lesssim 10^{-3}$.

Post common envelope binaries from SDSS

We have known for a long time that many of the measured white dwarf (WD) masses in cataclysmic variables (CVs) significantly exceed the mean mass of single WDs. This was thought to be related to observational biases, but recent high-precision measurements of WD masses in a great number of CVs are challenging this interpretation. We review the measured WD masses of CVs, determine the WD-mass distribution of an extensive sample of post-common-envelope binaries (PCEBs) that are representative for the progenitors of the current CV population (pre-CVs) and compare both distributions. We calculate the CV formation time of the PCEBs in our sample by determining the post common-envelope (CE) and the main-sequence evolution of the binary systems and define a pre-CV to be a PCEB that evolves into a semi-detached configuration with stable mass transfer within less than the age of the Galaxy. Possible observational biases affecting the WD-mass distribution for the pre-CV and the CV samples are discussed. The mean WD mass among CVs is <Mwd>=0.83\pm0.23 Msun, much larger than that found for pre-CVs, <Mwd>=0.67\pm0.21 Msun. Selection effects cannot explain the high WD masses observed in CVs and we here suggest two possible explanations, both of which imply substantial revisions to the standard model of CV evolution: either most CVs have formed above the orbital-period gap (which requires a high WD mass to initiate stable mass transfer or a previous phase of thermal-timescale mass transfer), or the mass of the WDs in CVs grows through accretion (which strongly disagrees with the predictions of classical nova models). Both options may imply that CVs contribute to the single-degenerate progenitors of Type Ia supernovae. The number of He-core WDs found in CVs (<=10%) is roughly consistent with the number of He-core WDs in pre-CVs (<=17+-8%), which indicates a low value of the CE efficiency.

The orbital period and system parameters of the recurrent nova T Pyx

T Pyx is a luminous recurrent nova that accretes at a much higher rate than is expected for its photometrically determined orbital period of about 1.8 hours. We here provide the first spectroscopic confirmation of the orbital period, P =1.829 hours (f=13.118368(11) c/d), based on time-resolved optical spectroscopy obtained at the VLT and the Magellan telescopes. We also derive an upper limit of the velocity semi-amplitude of the white dwarf, K1 = 17.9 +/- 1.6 km/s, and estimate a mass ratio of q = 0.20 +/- 0.03. If the mass of the donor star is estimated using the period-density relation and theoretical main-sequence mass-radius relation for a slightly inflated donor star, we find M2 = 0.14 +/- 0.03 Msun. This implies a mass of the primary white dwarf M1 = 0.7 +/- 0.2 Msun. If the white-dwarf mass is > 1 Msun, as classical nova models imply, the donor mass must be even higher. We therefore rule out the possibility that T Pyx has evolved beyond the period minimum for cataclysmic variables. We find that the system inclination is constrained to be approximately 10 degrees, confirming the expectation that T Pyx is a low-inclination system. We also discuss some of the evolutionary implications of the emerging physical picture of T Pyx. In particular, we show that epochs of enhanced mass transfer (like the present) may accelerate or even dominate the overall evolution of the system, even if they are relatively short-lived. We also point out that such phases may be relevant to the evolution of cataclysmic variables more generally.

A SEMI-EMPIRICAL MASS-LOSS RATE IN SHORT-PERIOD CATACLYSMIC VARIABLES

The mass-loss rate of donor stars in cataclysmic variables (CVs) is of paramount importance in the evolution of short-period CVs. Observed donors are oversized in comparison with those of isolated single stars of the same mass, which is thought to be a consequence of the mass loss. Using the empirical mass-radius relation of CVs and the homologous approximation for changes in effective temperature T_2, orbital period P, and luminosity of the donor with the stellar radius, we find the semi-empirical mass-loss rate M2_dot of CVs as a function of P. The derived M2_dot is at ~10^(-9.5)-10^(-10) Msun/yr and depends weakly on P when P > 90 min, while it declines very rapidly towards the minimum period when P < 90 min, emulating the P-T_2 relation. Due to strong deviation from thermal equilibrium caused by the mass loss, the semi-empirical M2_dot is significantly different from, and has a less-pronounced turnaround behavior with P than suggested by previous numerical models. The semi-empirical P-M2_dot relation is consistent with the angular momentum loss due to gravitational wave emission, and strongly suggests that CV secondaries with 0.075 Msun < M_2 < 0.2 Msun are less than 2 Gyrs old. When applied to selected eclipsing CVs, our semi-empirical mass-loss rates are in good agreement with the accretion rates derived from the effective temperatures T_1 of white dwarfs, suggesting that M2_dot can be used to reliably infer T_2 from T_1. Based on the semi-empirical M2_dot, SDSS 1501 and 1433 systems that were previously identified as post-bounce CVs have yet to reach the minimal period.

Two new systems with enhanced UV line flux ratios as indication of the evolution of cataclysmic variables

We present ultraviolet spectra of 62 cataclysmic variables observed with Hubble Space Telescope Space Telescope Imaging Spectrograph & Faint Object Spectrograph (HST STIS FOS) to diagnose cataclysmic variables with enhanced NV/C IV line flux ratios. We concentrated on calculating the line flux ratios of the above mentioned sample. We found that a number of cataclysmic variables reveal enhanced NV/C IV line flux ratios in agreement with the previous studies ( de Martino and Gänsicke, 2009; Gänsicke et al., 2003). Also we found that two new systems (U SCO, V1974 CYG) reveal enhanced NV/C IV line flux ratios. Such anomalous line flux ratios confirmed that these CVs went through a phase of thermal timescale mass transfer (TTSMT) and now accrete CNO processed material from a companion stripped of its external layers ( Gänsicke et al., 2003; Gänsicke, 2004; de Martino and Gänsicke, 2009).

Dwarf Novae in the Shortest Orbital Period Regime: II. WZ Sge Stars as the Missing Population near the Period Minimum

Abstract WZ Sge-type dwarf novae are characterized by long recurrence times of outbursts ($\sim$ 10 yr) and short orbital periods ($\lesssim$ 85 min). A significant part of WZ Sge stars may remain undiscovered because of low outburst activity. Recently, the observed orbital period distribution of cataclysmic variables (CVs) has changed partly because outbursts of new WZ Sge stars have been discovered routinely. Hence, estimations of the intrinsic population of WZ Sge stars are important to study the population and the evolution of CVs. In this paper, we present a Bayesian approach to estimate the intrinsic period distribution of dwarf novae from observed samples. In this Bayesian model, we assumed a simple relationship between the recurrence time and the orbital period, which is consistent with observations of WZ Sge stars and other dwarf novae. As a result, the minimum orbital period was estimated to be $\sim $ 70 min. The population of WZ Sge stars exhibited a spike-like feature at the shortest period regime in the orbital period distribution. These features are consistent with the orbital period distribution previously predicted by population synthesis studies. We propose that WZ Sge stars and CVs with a low mass-transfer rate are excellent candidates for the missing population predicted by the evolution theory of CVs.

Survey of Period Variations of Superhumps in SU UMa-Type Dwarf Novae

Abstract We systematically surveyed period variations of superhumps in SU UMa-type dwarf novae based on newly obtained data and past publications. In many systems, the evolution of the superhump period is found to be composed of three distinct stages: an early evolutionary stage with a longer superhump period, a middle stage with systematically varying periods, and a final stage with a shorter, stable superhump period. During the middle stage, many systems with superhump periods of less than 0.08 d show positive period derivatives. We present observational characteristics of these stages and give greatly improved statistics. Contrary to an earlier claim, we found no clear evidence for a variation of period derivatives among different superoutbursts of the same object. We present an interpretation that the lengthening of the superhump period is a result of the outward propagation of an eccentricity wave, which is limited by the radius near the tidal truncation. We interpret that late-stage superhumps are rejuvenated excitation of a 3:1 resonance when superhumps in the outer disk are effectively quenched. The general behavior of the period variation, particularly in systems with short orbital periods, appears to follow a scenario proposed in Kato, Maehara, and Monard (2008, PASJ, 60, L23). We also present an observational summary of WZ Sge-type dwarf novae. Many of them have shown long-enduring superhumps during a post-superoutburst stage having longer periods than those during the main superoutburst. The period derivatives in WZ Sge-type dwarf novae are found to be strongly correlated with the fractional superhump excess, or consequently with the mass ratio. WZ Sge-type dwarf novae with a long-lasting rebrightening or with multiple rebrightenings tend to have smaller period derivatives, and are excellent candidates for those systems around or after the period minimum of evolution of cataclysmic variables.

CATACLYSMIC VARIABLE PRIMARY EFFECTIVE TEMPERATURES: CONSTRAINTS ON BINARY ANGULAR MOMENTUM LOSS

We review the most decisive currently available measurements of the surface effective temperatures, Teff, of white dwarf (WD) primaries in cataclysmic variables (CVs) during accretion quiescence, and use these as a diagnostic for their time averaged accretion rate, <Mdot>. Using time-dependent calculations of the WD envelope, we investigate the sensitivity of the quiescent Teff to long term variations in the accretion rate. We find that the quiescent Teff provides one of the best available tests of predictions for the angular momentum loss and resultant mass transfer rates which govern the evolution of CVs. While gravitational radiation is sufficient to explain the <Mdot> of strongly magnetic CVs at all Porb, faster angular momentum loss is required by the temperatures of dwarf nova primaries (non-magnetic systems). This provides evidence that a normal stellar magnetic field structure near the secondary is essential for the enhanced braking mechanism to work, supporting the well-known stellar wind braking hypothesis. The contrast in <Mdot> is most prominent for orbital periods Porb > 3 hours, above the period gap, but a modest enhancement is also present at shorter Porb. The averaging time which <Mdot> reflects is as much as 10^5 years for low-<Mdot> systems and as little as 10^3 years for high-<Mdot> systems. We discuss the security of conclusions drawn about the CV population in light of these time scales and our necessarily incomplete sample of systems. Measurements for non-magnetic systems above the period gap fall below predictions from traditional stellar wind braking prescriptions, but above more recent predictions with somewhat weaker angular momentum loss. We also discuss the apparently high Teff's found in the VY Scl stars. (abridged)

An Hα-selected sample of cataclysmic variables – II. Implications for CV evolution

We use an independent new sample of cataclysmic variables (CVs), constructed by selecting objects for Ha emission, to constrain the properties of the intrinsic CV population. This sample is restricted to systems that are likely to be non-magnetic and unevolved; it consists of 17 CVs, of which at least 10 have orbital periods above 3 h. We find that even very generous allowance for selection effects is not sufficient to reconcile the large ratio of short- to long-period CVs predicted by standard CV evolution theory with the observed sample, possibly implying that short-period systems evolve faster than predicted by the disrupted magnetic braking model. This would require that an angular momentum-loss mechanism, besides gravitational radiation, acts on CVs with orbital periods below the period gap. To bring the model into agreement with observations, the rate of angular momentum loss below the period gap must be increased by a factor of at least 3, unless the model also over estimates the angular momentum-loss rate of long-period CVs.

An Hα-selected sample of cataclysmic variables – I. Observations of newly discovered systems

Strong selection effects are present in observational samples of cataclysmic variables (CVs), complicating comparisons to theoretical predictions. The selection criteria used to define most CV samples discriminate heavily against the discovery of short-period, intrinsically faint systems. The situation can be improved by selecting CVs for the presence of emission lines. For this reason, we have constructed a homogeneous sample of CVs selected on the basis of H? emission. We present discovery observations of the 14 CVs and two additional CV candidates found in this search. The orbital periods of 11 of the new CVs were measured; all are above 3 h. There are two eclipsing systems in the sample, and one in which we observed a quasi-periodic modulation on a ?1000 s time-scale. We also detect the secondary star in the spectrum of one system, and measure its spectral type. Several of the new CVs have the spectroscopic appearance of nova-like variables, and a few display what may be SW Sex star behaviour. In a companion paper, we discuss the implications of this new sample for CV evolution.

SW Sextantis stars: the dominant population of cataclysmic variables with orbital periods between 3 and 4 h

We present time-series optical photometry of five new cataclysmic variables (CVs) identified by the Hamburg Quasar Survey (HQS). The deep eclipses observed in HS 0129+2933 (= TT Tri), HS 0220+0603 and HS 0455+8315 provided very accurate orbital periods of 3.35129827(65), 3.58098501(34) and 3.56937674(26) h, respectively. HS 0805+3822 shows grazing eclipses and has a likely orbital period of 3.2169(2) h. Time-resolved optical spectroscopy of the new CVs (with the exception of HS 0805+3822) is also presented. Radial velocity studies of the Balmer emission lines provided an orbital period of 3.55 h for HS 1813+6122, which allowed us to identify the observed photometric signal at 3.39 h as a negative superhump wave. The spectroscopic behaviour exhibited by all the systems clearly identifies them as new SW Sextantis (SW Sex) stars. HS 0220+0603 shows unusual N ii and Si ii emission lines suggesting that the donor star may have experienced nuclear evolution via the CNO cycle. These five new additions to the class increase the number of known SW Sex stars to 35. Almost 40 per cent of the total SW Sex population do not show eclipses, invalidating the requirement of eclipses as a defining characteristic of the class and the models based on a high orbital inclination geometry alone. On the other hand, as more SW Sex stars are identified, the predominance of orbital periods in the narrow 3–4.5 h range is becoming more pronounced. In fact, almost half the CVs which populate the 3–4.5 h period interval are definite members of the class. The dominance of SW Sex stars is even stronger in the 2–3 h period gap, where they make up 55 per cent of all known gap CVs. These statistics are confirmed by our results from the HQS CVs. Remarkably, 54 per cent of the Hamburg nova-like variables have been identified as SW Sex stars with orbital periods in the 3–4.5 h range. The observation of this pile-up of systems close to the upper boundary of the period gap is difficult to reconcile with the standard theory of CV evolution, as the SW Sex stars are believed to have the highest mass-transfer rates among CVs. Finally, we review the full range of common properties that the SW Sex stars exhibit. Only a comprehensive study of this rich phenomenology will prompt to a full understanding of the phenomenon and its impact on the evolution of CVs and the accretion processes in compact binaries in general.