Period Bouncers Research Articles

Magnetic braking below the cataclysmic variable period gap and the observed dearth of period bouncers

Period bouncers are cataclysmic variables (CVs) that have evolved past their orbital period minimum. The strong disagreement between theory and observations of the relative fraction of period bouncers is a severe shortcoming in the understanding of CV evolution. We test the implications of the hypothesis that magnetic braking (MB), which is suggested to be an additional angular momentum loss (AML) mechanism for CVs below the period gap ($P_ orb 120$ min), weakens around their period minimum. We computed the evolution of CV donors below the period gap using the MESA code, assuming that the evolution of the system is driven by AML black due to gravitational wave radiation (GWR) and MB. We parametrised the MB strength as $ AML_ MB AML_ GWR black We computed two qualitatively different sets of models, one in which kappa is a constant and another in which kappa depends on stellar parameters in such a way that the value of kappa decreases as the CV approaches the period minimum ($P_ orb min), beyond which $ approx0$. We find that black two crucial effects drive the latter set of models. (1) A decrease in kappa as CVs approach the period minimum stalls their evolution so that they spend a long time in the observed period minimum spike ($80 P_ orb min Here, they become difficult to distinguish from pre-bounce systems in the spike. (2) A strong decrease in the mass-transfer rate makes them virtually undetectable as they evolve further. So, the CV stalls around the period minimum and then `disappears'. This reduces the number of detectable bouncers. Physical processes, such as dynamo action, white dwarf magnetism, and dead zones, may cause such a weakening of MB at short orbital periods. The weakening MB formalism black provides a possible solution to the problem of the dearth of black detectable period bouncers in CV observational surveys.

A joint SRG/eROSITA + ZTF search: Discovery of a 97-min period eclipsing cataclysmic variable with evidence of a brown dwarf secondary

ABSTRACT Cataclysmic variables (CVs) that have evolved past the period minimum during their lifetimes are predicted to be systems with a brown dwarf donor. While population synthesis models predict that around 40–70 per cent of the Galactic CVs are post-period minimum systems referred to as ‘period bouncers’, only a few dozen confirmed systems are known. We report the study and characterization of a new eclipsing CV, SRGeJ041130.3+685350 (SRGeJ0411), discovered from a joint SRG/eROSITA and ZTF programme. The optical spectrum of SRGeJ0411 shows prominent hydrogen and helium emission lines, typical for CVs. We obtained optical high-speed photometry to confirm the eclipse of SRGeJ0411 and determine the orbital period to be Porb ≈ 97.530 min. The spectral energy distribution suggests that the donor has an effective temperature of ≲ 1800 K. We constrain the donor mass with the period–density relationship for Roche lobe-filling stars and find that Mdonor ≲ 0.04 M⊙. The binary parameters are consistent with evolutionary models for post-period minimum CVs, suggesting that SRGeJ0411 is a new period bouncer. The optical emission lines of SRGeJ0411 are single-peaked despite the system being eclipsing, which is typically only seen due to stream-fed accretion in polars. X-ray spectroscopy hints that the white dwarf in SRGeJ0411 could be magnetic, but verifying the magnetic nature of SRGeJ0411 requires further investigation. The lack of optical outbursts has made SRGeJ0411 elusive in previous surveys, and joint X-ray and optical surveys highlight the potential for discovering similar systems in the near future.

Period bouncers as detached magnetic cataclysmic variables

Context. The general prediction that more than half of all cataclysmic variables (CVs) have evolved past the period minimum is in strong disagreement with observational surveys, which show that the relative number of these objects is just a few percent. Aims. Here, we investigate whether a large number of post-period minimum CVs could detach because of the appearance of a strong white dwarf magnetic field potentially generated by a rotation- and crystallization-driven dynamo. Methods. We used the MESA code to calculate evolutionary tracks of CVs incorporating the spin evolution and cooling as well as compressional heating of the white dwarf. If the conditions for the dynamo were met, we assumed that the emerging magnetic field of the white dwarf connects to that of the companion star and incorporated the corresponding synchronization torque, which transfers spin angular momentum to the orbit. Results. We find that for CVs with donor masses exceeding ∼0.04 M⊙, magnetic fields are generated mostly if the white dwarfs start to crystallize before the onset of mass transfer. It is possible that a few white dwarf magnetic fields are generated in the period gap. For the remaining CVs, the conditions for the dynamo to work are met beyond the period minimum, when the accretion rate decreased significantly. Synchronization torques cause these systems to detach for several gigayears even if the magnetic field strength of the white dwarf is just one MG. Conclusions. If the rotation- and crystallization-driven dynamo – which is currently the only mechanism that can explain several observational facts related to magnetism in CVs and their progenitors – or a similar temperature-dependent mechanism is responsible for the generation of magnetic field in white dwarfs, most CVs that have evolved beyond the period minimum must detach for several gigayears at some point. This reduces the predicted number of semi-detached period bouncers by up to ∼60 − 80%.

The period bouncer system SDSS J105754.25+275947.5: First radial velocity study

Abstract We report the first radial velocity spectroscopic study of the eclipsing period bouncer SDSS J105754.25+275947.5. Together with eclipse light curve modeling, we redetermined the system parameters and studied the accretion disk structure. We confirm that the system contains a white dwarf with MWD = 0.83(3) M⊙ and an effective temperature of 11 500(400) K. The mass of the secondary is M2 = 0.056 M⊙ with an effective temperature of T2 = 2100 K or below. The system inclination is i = 84$fdg$3(6). The data is in good agreement with our determination of K1 = 33(4) km s−1. We estimate the mass transfer rate as $\dot{M}=1.9(2)\times 10^{-11}$M⊙yr −1. Based on an analysis of the SDSS and OSIRIS spectra, we conclude that the optical continuum is formed predominantly by the radiation from the white dwarf. The contribution of the accretion disk is low and originates from the outer part of the disk. The Balmer emission lines are formed in a plasma with log N0 = 12.7 [cm−1] and a kinetic temperature of T∼ 10,000 K. The size of the disk, where the emission lines are formed, expands up to Rd, out = 0.29 R⊙. The inner part of the emission line forming region goes down to Rd, in ≈ 2RWD . The Doppler tomography and trailed spectra shows the presence of a hot spot and a clumpy structure in the disk, with variable intensity along the disk position angle. There is an extended region at the side opposite the hot spot with two bright clumps caused more probably by non-Keplerian motion there.

Cataclysmic Variables from Sloan Digital Sky Survey – V. The search for period bouncers continues

ABSTRACT SDSS-V is carrying out a dedicated survey for white dwarfs, single, and in binaries, and we report the analysis of the spectroscopy of 118 cataclysmic variables (CVs) and CV candidates obtained during the final plug plate observations of SDSS. We identify eight new CVs, spectroscopically confirm 53 and refute 11 published CV candidates, and we report 21 new or improved orbital periods. The orbital period distribution of the SDSS-V CVs does not clearly exhibit a period gap. In common with previous studies, the distribution shows that spectroscopically identified CVs have a larger proportion of short-period systems compared to samples identified from photometric variability. Remarkably, despite a systematic search, we find very few period bouncers. We estimate the space density of period bouncers to be $\simeq 0.2\times 10^{-6}\, \mathrm{pc}^{-3}$, i.e. they represent only a few per cent of the total CV population. This suggests that during their final phase of evolution, CVs either destroy the donor, e.g. via a merger, or that they become detached and cease mass transfer.

A catalogue of cataclysmic variables from 20 yr of the Sloan Digital Sky Survey with new classifications, periods, trends, and oddities

ABSTRACT We present a catalogue of 507 cataclysmic variables (CVs) observed in SDSS I to IV including 70 new classifications collated from multiple archival data sets. This represents the largest sample of CVs with high-quality and homogeneous optical spectroscopy. We have used this sample to derive unbiased space densities and period distributions for the major sub-types of CVs. We also report on some peculiar CVs, period bouncers and also CVs exhibiting large changes in accretion rates. We report 70 new CVs, 59 new periods, 178 unpublished spectra, and 262 new or updated classifications. From the SDSS spectroscopy, we also identified 18 systems incorrectly identified as CVs in the literature. We discuss the observed properties of 13 peculiar CVS, and we identify a small set of eight CVs that defy the standard classification scheme. We use this sample to investigate the distribution of different CV sub-types, and we estimate their individual space densities, as well as that of the entire CV population. The SDSS I to IV sample includes 14 period bounce CVs or candidates. We discuss the variability of CVs across the Hertzsprung–Russell diagram, highlighting selection biases of variability-based CV detection. Finally, we searched for, and found eight tertiary companions to the SDSS CVs. We anticipate that this catalogue and the extensive material included in the Supplementary Data will be useful for a range of observational population studies of CVs.

Period Bouncer Cataclysmic Variable EZ Lyn in Quiescence

Abstract We report the study of the accretion disk structure of the period bouncer cataclysmic variable EZ Lyn (SDSS J080434.20+510349.2) in quiescence based on our new time-resolved photometric and spectroscopic observations and data extracted from archives. The object magnitude now is V = 17.95(5), close to its brightest before the first superoutburst in 2006. We confirmed the presence of the small eclipse in the optical light curve. The spectra obtained in quiescence at different epochs look similar. However, the contribution of the disk and intensities of emission lines are strongly varied. We singled out pure accretion disk spectra and found that the Balmer decrement Hα:Hβ:Hγ:Hδ = 1.61:1.0:0.76:0.59 is comparable with one at bright accretion disks in longer period cataclysmic variables. The decrement suggests that emission lines are excited collisionally in an optical thin part of the disk with average density and temperature of logN 0 = 12.5(2) and T = (10–15) × 103 K. Based on the photometric data and our modeling techniques, we redetermined the mass of M WD = 0.85(1) M ⊙ and the current effective temperature T WD,eff = 11,250(50) K of the white dwarf. The secondary has mass of M 2 = 0.042(14) M ⊙. The system inclination is 79.0°(2). The mass accretion rate is about M ̇ ≈ ( 0.3 – 3.0 ) × 10 − 12 M ⊙ yr−1. The disk luminosity, together with results from light-curve modeling, suggests a low effective temperature ∼2500 K of the continuum emitting region, where also the spiral arm pattern is hosted.

The magnetic system SMSS J1606−1000 as a period bouncer

ABSTRACT We report the discovery of a rare close binary system, SMSS J160639.78−100010.7, comprising a magnetic white dwarf with a field of about 30 MG and a brown dwarf. We measured an orbital period of 92 min which is consistent with the photometric period. Minimum and maximum light occur at the orbital quadratures Φ = 0.25 and 0.75, respectively, and cannot be caused by reflection on the brown dwarf, but, instead, by a spot on the synchronously rotating magnetic white dwarf. The brown dwarf does not fill its Roche lobe and the system may be in a low-accretion state or, more likely, in a detached state following episodes of mass transfer. SMSS J160639.78−100010.7 is the nearest known magnetic white dwarf plus brown dwarf system.

X-ray and ultraviolet observations of the eclipsing cataclysmic variables OV Bootis and SDSS J103533.02+055158.3 with degenerate donors

Context.The majority of cataclysmic variables are predicted to be post-period minimum systems with degenerate donor stars, the period bouncers. Owing to their intrinsic faintness, however, only a handful of these systems have so far been securely identified.Aims.We want to study the X-ray properties of two eclipsing period bouncers, OV Bootis and SDSS J103533.02+055158.3, that were selected for this study due to their proximity to Earth.Methods.We have obtainedXMM-Newtonphase-resolved X-ray and ultraviolet observations of the two objects for spectral and timing analysis.Results.Owing to a recent dwarf nova outburst OV Bootis was much brighter than SDSS J103533.02+055158.3 at X-ray and ultraviolet wavelengths and the eclipse could be studied in some detail. An updated eclipse ephemeris was derived. The X-rays were shown to originate close to the white dwarf, the boundary layer, with significant absorption affecting its spectrum. There was no absorption in SDSS J103533.02+055158.3, despite being observed at the same inclination indicating different shapes of the disk and the disk rim. The white-dwarf temperature was re-determined for both objects: the white dwarf in OV Bootis was still hot (23 000 K) five months after a dwarf nova outburst, and the white dwarf in SDSS J103533.02+055158.3 hotter than assumed previously (Teff = 11 500 K).Conclusions.All three cataclysmic variables with degenerate donors studied so far in X-rays, including SDSS J121209.31+013627.7, were clearly discovered in X-rays and revealed mass accretion ratesṀ= 8 × 10−15− 4 × 10 − 13M⊙yr−1. If their X-ray behavior is representative of the subpopulation of period bouncers, the all-sky X-ray surveys with eROSITA together with comprehensive follow-up will uncover new objects in sufficient number to address the remaining questions concerning late-stage cataclysmic variable evolution.

Multi-wavelength photometry during the 2018 superoutburst of the WZ Sge-type dwarf nova EG Cancri

Abstract We report on the multi-wavelength photometry of the 2018 superoutburst in EG Cnc. We have detected stage A superhumps and long-lasting late-stage superhumps via the optical photometry and have constrained the binary mass ratio and its possible range. The median value of the mass ratio is 0.048 and the upper limit is 0.057, which still implies that EG Cnc is one of the possible candidates for period bouncers. This object also showed multiple rebrightenings in this superoutburst which are the same as those in its previous superoutburst in 1996–1997, despite the difference in the main superoutburst. This would represent that the rebrightening type is inherent to each object and is independent of the initial disk mass at the beginning of superoutbursts. We also found that B − I and J − Ks colors were unusually red just before the rebrightening phase and became bluer during the quiescence between rebrightenings, which would mean that the low-temperature mass reservoir at the outermost disk accreted with time after the main superoutburst. Also, the ultraviolet flux was sensitive to rebrightenings as well as the optical flux, and the U − B color became redder during the rebrightening phase, which would indicate that the inner disk became cooler when this object repeated rebrightenings. Our results thus basically support the idea that the cool mass reservoir in the outermost disk is responsible for rebrightenings.

The Compact binary HIgh CAdence Survey (CHiCaS): An overview

Cataclysmic Variables (CVs) are binaries in which a white dwarf accretes from a low-mass companion star. CVs are the best-suited laboratories to test our understanding of the evolution of compact, interacting binaries as they are numerous, relatively bright, and both stellar components are structurally simple. Nonetheless, while a large fraction (≃40-80%) of the present-day Galactic CV population is expected to be old, highly evolved and to host brown dwarf companions, yet very few of these so-called “period bouncers” have been identified so far. The lack of these systems in the observed Galactic CV population possibly suggests that the physical mechanisms driving CV evolution (such as the common envelope phase, the mechanisms of angular momentum loss and/or the response of the companions to the mass loss) are still not completely understood.The Compact binary HIgh CAdence Survey (CHiCaS) is a high cadence photometric survey performing three hours of uninterrupted time series photometry at one minute cadence over 136 square degrees of sky with JAST/T80Cam. CHiCaS aims to unambiguously identify the predicted large population of period-bounce CVs via detection of binary eclipses, thus providing an observational support for the current evolutionary models of all kind of compact binaries, such as black hole binaries, X-ray transients, double degenerates or SN Ia progenitors. Moreover, CHiCaS will deliver high cadence light curves along with full-colour information for about 2.5 million sources, thus identifying several hundred thousand variable stars, including eclipsing and contact binaries, pulsating and flaring stars, which will provide a significant legacy value.

Evidence for reduced magnetic braking in polars from binary population models

ABSTRACT We present the first population synthesis of synchronous magnetic cataclysmic variables, called polars, taking into account the effect of the white dwarf (WD) magnetic field on angular momentum loss. We implemented the reduced magnetic braking (MB) model proposed by Li, Wu & Wickramasinghe into the Binary Stellar Evolution (bse) code recently calibrated for cataclysmic variable (CV) evolution. We then compared separately our predictions for polars and non-magnetic CVs with a large and homogeneous sample of observed CVs from the Sloan Digital Sky Survey. We found that the predicted orbital period distributions and space densities agree with the observations if period bouncers are excluded. For polars, we also find agreement between predicted and observed mass transfer rates, while the mass transfer rates of non-magnetic CVs with periods ≳3 h drastically disagree with those derived from observations. Our results provide strong evidence that the reduced MB model for the evolution of highly magnetized accreting WDs can explain the observed properties of polars. The remaining main issues in our understanding of CV evolution are the origin of the large number of highly magnetic WDs, the large scatter of the observed mass transfer rates for non-magnetic systems with periods ≳3 h, and the absence of period bouncers in observed samples.

The cataclysmic variable QZ Lib: a period bouncer

While highly evolved cataclysmic variables (CVs) with brown dwarf donors, often called "period bouncers", are predicted to make up $\simeq40-70$ % of the Galactic CV population, only a handful of such systems are currently known. The identification and characterization of additional period bouncers is therefore important to probe this poorly understood phase of CV evolution. We investigate the evolution of the CV QZ Lib following its 2004 super-outburst using multi-epoch spectroscopy. From time-resolved spectroscopic observations we measure the orbital period of the system, $P_\mathrm{orb}= 0.06436(20)$ d, which, combined with the superhump period $P_\mathrm{SH}= 0.064602(24)$ d, yields the system mass ratio, $q = 0.040(9)$. From the analysis of the spectral energy distribution we determine the structure of the accretion disc and the white dwarf effective temperature, $T_\mathrm{eff} = 10\,500 \pm 1500\,\mathrm{K}$. We also derive an upper limit on the effective temperature of the secondary, $T_\mathrm{eff} \lesssim 1700\,\mathrm{K}$, corresponding to a brown dwarf of T spectral type. The low temperature of the white dwarf, the small mass ratio and the fact that the donor is not dominating the near-infrared emission are all clues of a post bounce system. Although it is possible that QZ Lib could have formed as a white dwarf plus a brown dwarf binary, binary population synthesis studies clearly suggest this scenario to be less likely than a period bouncer detection and we conclude that QZ Lib is a CV that has already evolved through the period minimum.

No cataclysmic variables missing: higher merger rate brings into agreement observed and predicted space densities

The predicted and observed space density of cataclysmic variables (CVs) have been for a long time discrepant by at least an order of magnitude. The standard model of CV evolution predicts that the vast majority of CVs should be period bouncers, whose space density has been recently measured to be $\rho \lesssim 2 \times 10^{-5}$ pc$^{-3}$. We performed population synthesis of CVs using an updated version of the Binary Stellar Evolution (BSE) code for single and binary star evolution. We find that the recently suggested empirical prescription of consequential angular momentum loss (CAML) brings into agreement predicted and observed space densities of CVs and period bouncers. To progress with our understanding of CV evolution it is crucial to understand the physical mechanism behind empirical CAML. Our changes to the BSE code are also provided in details, which will allow the community to accurately model mass transfer in interacting binaries in which degenerate objects accrete from low-mass main-sequence donor stars.

ASASSN-16dt and ASASSN-16hg: Promising candidate period bouncers

AbstractWe present optical photometry of superoutbursts that occurred in 2016 of two WZ Sge-type dwarf novae (DNe), ASASSN-16dt and ASASSN-16hg. Their light curves showed a dip in brightness between the first plateau stage with no ordinary superhumps (or early superhumps) and the second plateau stage with ordinary superhumps. We find that the dip is produced by the slow evolution of the 3 : 1 resonance tidal instability and that it would likely be observed in low mass-ratio objects. An estimated mass ratio (q ≡ M2/M1) from the period of developing (stage A) superhumps [0.06420(3) d] was 0.036(2) in ASASSN-16dt. Additionally, its superoutburst has many properties similar to those in other low-q WZ Sge-type DNe: long-lasting stage-A superhumps, small superhump amplitudes, long delay of ordinary-superhump appearances, and a slow decline rate in the plateau stage with superhumps. Its very small mass ratio and observational characteristics suggest that this system is one of the best candidates for a period bouncer—a binary accounting for the missing population of post-period minimum cataclysmic variables. Although it is not clearly verified due to the lack of detection of stage-A superhumps, ASASSN-16hg might be a possible candidate for period bouncers on the basis of the morphology of its light curves and the small superhump amplitudes. Many outburst properties of period bouncer candidates would originate from the small tidal effects of their secondary stars.

Survey of period variations of superhumps in SU UMa-type dwarf novae. IX. The ninth year (2016–2017)

Abstract Continuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 127 SU UMa-type dwarf novae observed mainly during the 2016–2017 season and characterized these objects. We provide updated statistics of the relation between the orbital period and the variation of superhumps, the relation between period variations and the rebrightening type in WZ Sge-type objects. We obtained the period minimum of 0.05290(2) d and confirmed the presence of the period gap above the orbital period ∼0.09 d. We note that four objects (NY Her, 1RXS J161659.5+620014, CRTS J033349.8−282244, and SDSS J153015.04+094946.3) have supercycles shorter than 100 d but show infrequent normal outbursts. We consider that these objects are similar to V503 Cyg, whose normal outbursts are likely suppressed by a disk tilt. These four objects are excellent candidates to search for negative superhumps. DDE 48 appears to be a member of ER UMa-type dwarf novae. We identified a new eclipsing SU UMa-type object, MASTER OT J220559.40−341434.9. We observed 21 WZ Sge-type dwarf novae during this interval and report 18 of them in this paper. Among them, ASASSN-16js is a good candidate for a period bouncer. ASASSN-16ia showed a precursor outburst for the first time in a WZ Sge-type superoutburst. ASASSN-16kg, CRTS J000130.5+050624, and SDSS J113551.09+532246.2 are located in the period gap. We have newly obtained 15 orbital periods, including periods from early superhumps.

Survey of period variations of superhumps in SU UMa-type dwarf novae. VIII. The eighth year (2015–2016)

AbstractContinuing the project described by Kato et al. (2009, PASJ, 61, S395), we collected times of superhump maxima for 128 SU UMa-type dwarf novae observed mainly during the 2015–2016 season and characterized these objects. The data have improved the distribution of orbital periods, the relation between the orbital period and the variation of superhumps, and the relation between period variations and the rebrightening type in WZ Sge-type objects. Coupled with new measurements of mass ratios using growing stages of superhumps, we now have a clearer and statistically greatly improved evolutionary path near the terminal stage of evolution of cataclysmic variables. Three objects (V452 Cas, KK Tel, and ASASSN-15cl) appear to have slowly growing superhumps, which is proposed to reflect the slow growth of the 3 : 1 resonance near the stability border. ASASSN-15sl, ASASSN-15ux, SDSS J074859.55+312512.6, and CRTS J200331.3−284941 are newly identified eclipsing SU UMa-type (or WZ Sge-type) dwarf novae. ASASSN-15cy has a short (∼0.050 d) superhump period and appears to belong to EI Psc-type objects with compact secondaries having an evolved core. ASASSN-15gn, ASASSN-15hn, ASASSN-15kh, and ASASSN-16bu are candidate period bouncers with superhump periods longer than 0.06 d. We have newly obtained superhump periods for 79 objects and 13 orbital periods, including periods from early superhumps. In order that future observations will be more astrophysically beneficial and rewarding to observers, we propose guidelines on how to organize observations of various superoutbursts.

ASASSN-15jd: WZ Sge-type star with intermediate superoutburst between single and double ones

Abstract We present optical photometry of a WZ Sge-type dwarf nova (DN), ASASSN-15jd. Its light curve showed a small dip in the middle of the superoutburst in 2015 for the first time among WZ Sge-type DNe. The unusual light curve implies a delay in the growth of the 3 : 1 resonance tidal instability. Also, the light curve is similar to those of two other WZ Sge-type stars, SSS J122221.7−311523 and OT J184228.1+483742, which are believed to be the best candidates for period bouncers on the basis of their small values of the mass ratio (q ≡ M2/M1). Additionally, the small mean superhump amplitude (<0.1 mag) and the long duration of no ordinary superhumps at the early stage of its superoutburst are common to the best candidates for period bouncers. Its average superhump period was Psh = 0.0649810(78) d and no early superhumps were detected. Although we could not estimate a mass ratio of ASASSN-15jd with high accuracy, this object is expected to be a candidate for a period bouncer—a binary accounting for the missing population of post-period minimum cataclysmic variables—based on the above characteristics.

WZ Sge-type dwarf novae

AbstractWe have summarized the current understanding and recently obtained findings about WZ Sge-type dwarf novae. We also reviewed the historical development of the understanding of these objects, provided the modern criteria, and reviewed the past research in relation to superhumps, early superhumps, and the outburst mechanism. We regard the presence of early superhumps (reflecting the 2:1 resonance) and long or multiple rebrightenings as the best distinguishing properties of WZ Sge-type dwarf novae. We provided the updated list of nearly 100 WZ Sge-type dwarf novae mainly based on the data obtained by the VSNET Collaboration up to Kato et al. (2015, PASJ, 67, 105), and discussed the statistics. We could detect early superhumps with amplitude larger than 0.02 mag in 63% of the studied WZ Sge-type dwarf novae, which makes early superhumps a useful distinguishing feature for WZ Sge-type dwarf novae. Theoretical light curves of early superhumps generally appear to reproduce the existence of many low-amplitude objects, supporting the geometrical origin of early superhumps. Using the recently developed method of measuring mass ratios using developing phase of superhumps (stage A superhumps), we showed that there is a linear relation between the period variation of superhumps and the mass ratio in WZ Sge-type objects. By using this relation, we were able to draw an evolutionary picture of a large number of WZ Sge-type dwarf novae and identified the type of outburst to be an evolutionary sequence: type C → D → A → B → E, with some outliers for type-B objects. The duration of stage A (evolutionary phase) of superhumps is also well correlated with the estimated mass ratios. By using mass ratios from stage A superhumps and the duration of stage A, we have been able to identify the best candidates for period bouncers.

OT J075418.7+381225 and OT J230425.8+062546: Promising candidates for the period bouncer

Abstract We report on photometric observations of two dwarf novae, OT J075418.7+381225 and OT J230425.8+062546, which showed superoutbursts in 2013 (OT J075418) and in 2011 (OT J230425). Their mean periods of the superhump were 0.0722403(26) d (OT J075418) and 0.067317(35) d (OT J230425). These objects showed a very long growth stage of the superhump (stage A) and a large period decrease in the stage A–B transition. The long stage A suggests slow evolution of the superhump due to the very small mass ratio of these objects. The declining rates during the plateau phase in the superoutburst of these objects were lower than those of SU UMa-type dwarf novae (DNe) with a similar superhump period. These properties were similar to those of SSS J122221.7−311523, the most likely candidate for the period bouncer. Therefore, these two DNe are regarded as likely candidates for the period bouncer. We estimated the number density of period bouncers roughly from our observations for the last five years. There is a possibility that these WZ Sge-type DNe with unusual outburst properties might account for the missing population of the period bouncer suggested by the evolutionary scenario.