GP Com Research Articles

Close-in Exoplanets as Candidates for Strange Quark Matter Objects

Abstract Since the true ground state of hadrons may be strange quark matter (SQM), pulsars may actually be strange stars rather than neutron stars. According to this SQM hypothesis, strange planets can also stably exist. The density of normal matter planets can hardly be higher than 30 g cm−3. They will be tidally disrupted when the orbital radius is less than ∼5.6 × 1010 cm, or when the orbital period (P orb) is less than ∼6100 s. However, an SQM planet can safely survive even when it is very close to the host, due to its high density. This feature can help us identify SQM objects. Here, we have tried to search for SQM objects among close-in exoplanets orbiting around pulsars. It is found that four pulsar planets (XTE J1807-294 b, XTE J1751-305 b, PSR 0636 b, PSR J1807-2459A b) completely meet the criterion of P orb < 6100 s, and are thus good candidates for SQM planets. The periods of two other planets (PSR J1719-14 b and PSR J2051-0827 b) are only slightly larger than the criterion value. They could be regarded as potential candidates. Additionally, we find that the periods of five white dwarf planets (GP Com b, V396 Hya b, J1433 b, WD 0137-349 b, and SDSS J1411+2009 b) are less than 0.1 day; they might also be SQM planets. Gravitational wave emissions from these close-in planetary systems are calculated from the view of various gravitational wave detectors.

On the possibility of detecting ultrashort period exoplanets with LISA

ABSTRACT Cunha, Silva & Lima recently reexamined the possibility of detecting gravitational waves from exoplanets, claiming that three ultrashort period systems would be observable by LISA. We revisit their analysis and conclude that the currently known exoplanetary systems are unlikely to be detectable, even assuming a LISA observation time Tobs = 4 yr. Conclusive statements on the detectability of one of these systems, GP Com b, will require better knowledge of the system’s properties, as well as more careful modelling of both LISA’s response and theG alactic confusion noise. Still, the possibility of exoplanet detection with LISA is interesting enough to warrant further study, as gravitational waves could yield dynamical properties that are difficult to constrain with electromagnetic observations.

The double helium-white dwarf channel for the formation of AM CVn binaries

Most close double helium white dwarfs will merge within a Hubble time due to orbital decay by gravitational wave radiation. However, a significant fraction with low mass ratios will survive for a long time as a consequence of stable mass transfer. Such stable mass transfer between two helium white dwarfs (HeWDs) provides one channel for the production of AM CVn binary stars. In previous calculations of double HeWD progenitors, the accreting HeWD was treated as a point mass. We have computed the evolution of 16 double HeWD models in order to investigate the consequences of treating the evolution of both components in detail. We find that the boundary between binaries having stable and unstable mass transfer is slightly modified by this approach. By comparing with observed periods and mass ratios, we redetermine masses of eight known AM CVn stars by our double HeWDs channel, i.e. HM Cnc, AM CVn, V406 Hya, J0926, J1240, GP Com, Gaia14aae and V396 Hya.We propose that central spikes in the triple-peaked emission spectra of J1240, GP Com and V396 Hya and the surface abundance ratios of N/C/O in GP Com can be explained by the stable double HeWD channel. The mass estimates derived from our calculations are used to discuss the predicted gravitational wave signal in the context of the Laser Interferometer Space Antenna (LISA) project.

UVES and X-Shooter spectroscopy of the emission line AM CVn systems GP Com and V396 Hya

We present time-resolved spectroscopy of the AM CVn-type binaries GP Com and V396 Hya obtained with VLT/X-Shooter and VLT/UVES. We fully resolve the narrow central components of the dominant helium lines and determine radial velocity semi-amplitudes of $K_{\rm spike} = 11.7\pm0.3$ km s$^{-1}$ for GP Com and $K_{\rm spike} = 5.8\pm0.3$ km s$^{-1}$ for V396 Hya. The mean velocities of the narrow central components show variations from line to line. Compared to calculated line profiles that include Stark broadening we are able to explain the displacements, and the appearance of forbidden helium lines, by additional Stark broadening of emission in a helium plasma with an electron density $n_e\simeq 5\times 10^{15}$ cm$^{-3}$. More than $30$ nitrogen and more than $10$ neon lines emission lines were detected in both systems. Additionally, $20$ nitrogen absorption lines are only seen in GP Com. The radial velocity variations of these lines show the same phase and velocity amplitude as the central helium emission components. The small semi-amplitude of the central helium emission component, the consistency of phase and amplitude with the absorption components in GP Com as well as the measured Stark effect shows that the central helium emission component, the so-called central-spike, is consistent with an origin on the accreting white dwarf. We use the dynamics of the bright spot and the central spike to constrain the binary parameters for both systems and find a donor mass of $9.6$ - $42.8$ M$_{\rm Jupiter}$ for GP Com and $6.1$ - $30.5$ M$_{\rm Jupiter}$ for V396 Hya. We find an upper limit for the rotational velocity of the accretor of $v_{\rm rot}<46$ km s$^{-1}$ for GP Com and $v_{\rm rot}<59$ km s$^{-1}$ for V396 Hya which excludes a fast rotating accretor in both systems.

The AM Canum Venaticorum binary SDSS J173047.59+554518.5

The AM Canum Venaticorum (AM CVn) binaries are a rare group of hydrogen-deficient, ultrashort period, mass-transferring white dwarf binaries and are possible progenitors of Type Ia supernovae. We present time-resolved spectroscopy of the recently discovered AM CVn binary SDSS J173047.59+554518.5. The average spectrum shows strong double-peaked helium emission lines, as well as a variety of metal lines, including neon; this is the second detection of neon in an AM CVn binary, after the much brighter system GP Com. We detect no calcium in the accretion disc, a puzzling feature that has been noted in many of the longer period AM CVn binaries. We measure an orbital period, from the radial velocities of the emission lines, of 35.2 ± 0.2 min, confirming the ultracompact binary nature of the system. The emission lines seen in SDSS J1730 are very narrow, although double-peaked, implying a low-inclination, face-on accretion disc; using the measured velocities of the line peaks, we estimate i ≤ 11°. This low inclination makes SDSS J1730 an excellent system for the identification of emission lines.

THE HOT COMPONENTS OF AM CVn HELIUM CATACLYSMICS

We present the results of a multi-component synthetic spectral analysis of the archival far ultraviolet spectra of the hot components of several AM CVn double degenerate interacting binaries with known distances from trigonometric parallaxes. Our analysis was carried out using the code BINSYN (Linnell & Hubeny 1996) which takes into account the donor companion star, the shock front which forms at the disk edge and the FUV and NUV energy distribution. We fixed the distance of each system at its parallax-derived value and adopted appropriate values of orbital inclination and white dwarf mass. We find that the accretion-heated "DO/DB" WDs are contributing significantly to the FUV flux in four of the systems (ES Ceti, CR Boo, V803 Cen, HP Lib, GP Com). In two of the systems, GP Com and ES Ceti, the WD dominates the FUV/NUV flux. We present model-derived accretion rates which agree with the low end of the range of accretion rates derived earlier from black body fits over the entire spectral energy distribution. We find that the WD in ES Ceti is very likely not a direct impact accretor but has a small disk. The WD in ES Ceti has $T_{eff} \sim 40,000 \pm 10,000$K. This is far cooler than the previous estimate of Espaillat et al.(2005). We find that the WD in GP Com has $T_{eff} = 14,800 \pm500$K, which is hotter than the previously estimated temperature of 11,000K. We present a comparison between our empirical results and current theoretical predictions for these systems.

The chemical composition of donors in AM CVn stars and ultracompact X-ray binaries: observational tests of their formation

We study the formation of ultra-compact binaries (AM CVn stars and ultra-compact X-ray binaries) with emphasis on the surface chemical abundances of the donors in these systems. Hydrogen is not convincingly detected in the spectra of these systems. Three different proposed formation scenarios involve different donor stars, white dwarfs, helium stars or evolved main-sequence stars. Using detailed evolutionary calculations we show that the abundances of helium WD donors and evolved main-sequence stars are close to equilibrium CNO-processed material, and the detailed abundances correlate with the core temperature and thus mass of the MS progenitors. Evolved MS donors typically have traces of H left. For hybrid or CO white dwarf donors, the carbon and oxygen abundances depend on the temperature of the helium burning and thus on the helium core mass of the progenitors. For helium star donors in addition to their mass, the abundances depend strongly on the amount of helium burnt before mass transfer starts and can range from unprocessed and thus almost equal to CNO-processed matter, to strongly processed and thus C/O rich and N-deficient. We briefly discuss the relative frequency of these cases for helium star donors, based on population synthesis results. Finally we give diagnostics for applying our results to observed systems and find that the most important test is the N/C ratio, which can indicate the formation scenario as well as, in some cases, the mass of the progenitor of the donor. In addition, if observed, the N/O, O/He and O/C ratios can distinguish between helium star and WD donors. Applied to the known systems we find evidence for WD donors in the AM CVn systems GP Com, CE 315 and SDSS J0804+16 and evidence for hybrid WD or very evolved helium star donors in the UCXBs 4U 1626-67 and 4U 0614+09. [Abridged]

Hubble Space TelescopeParallaxes of AM CVn Stars and Astrophysical Consequences

We present absolute parallaxes and relative proper motions for five AM CVn stars, which we obtained using the Fine Guidance Sensors on board the Hubble Space Telescope. Our parallax measurements translate into distances dAM CVn = 606(-93)(+135) pc, d(HP) Lib = 197(-12)(+14) pc, d(CR) Boo = 337(-35)(+44) pc, d(V803) Cen(-27)(+32) pc, and d(GP) Com = 75(-2)(+2) pc. From these distances we estimate the space density of AM CVn stars and suggest that previous estimates have been too high by about an order of magnitude. We also infer the mass accretion rates, which allows us to constrain the masses of the donor stars, and we show that relatively massive, semidegenerate donor stars are favored in all systems except GP Com. Finally, we give updated estimates for the systems' gravitational wave signals, based on their distances and the inferred masses of the binary components, which are relevant for future space missions such as the proposed Laser Interferometer Space Antenna (LISA). We show that all systems but GP Com are excellent candidates for detection with LISA.

Accretion disc dynamics in extreme mass ratio compact binaries

An analysis is presented of a numerical investigation of the dynamics and geometry of accretion discs in binary systems with mass ratios q = M 2 /M 1 < 0.1, applicable to ultracompact X-ray binaries, AM CVn stars and very short period cataclysmic variables. The steady-state geometry of the disc in the binary reference frame is found to be quite different from that expected at higher mass ratios. For q ∼ 0.1, the disc takes on the usual elliptical shape, with the major axis aligned perpendicular to the line of centres of the two stars. However, at smaller mass ratios the elliptical gaseous orbits in the outer regions of the disc are rotated in the binary plane. The angle of rotation increases with gas temperature, but is found to vary inversely with q. At q = 0.01, the major axis of these orbits is aligned almost parallel to the line of centres of the two stars. These effects may be responsible for the similar disc structure inferred from Doppler tomography of the AM CVn star GP Com, which has q = 0.02. The steady-state geometry at low mass ratios is not predicted by an inviscid, restricted three-body model of gaseous orbits; it is related to the effects of tidal-viscous truncation of the disc near the Roche lobe boundary. Since the disc geometry can be inferred observationally for some systems, it is proposed that this may offer a useful diagnostic for the determination of mass ratios in ultracompact binaries.

Kinematics of the ultracompact helium accretor AM Canum Venaticorum

We report on the results from a five-night campaign of high-speed spectroscopy of the 17-min binary AM Canum Venaticorum (AM CVn), obtained with the 4.2-m William Herschel Telescope on La Palma. We detect a kinematic feature that appears to be entirely analogous to the 'central spike' known from the long-period, emission-line AM CVn stars GP Com, V396 Hya and SDSS J124058.03-015919.2, which has been attributed to the accreting white dwarf. Assuming that the feature indeed represents the projected velocity amplitude and phase of the accreting white dwarf, we derive a mass ratio q = 0.18 +/- 0.01 for AM CVn. This is significantly higher than the value found in previous, less direct measurements. We discuss the implications for AM CVn's evolutionary history and show that a helium star progenitor scenario is strongly favoured. We further discuss the implications for the interpretation of AM CVn's superhump behaviour, and for the detectability of its gravitational-wave signal with the Laser Interferometer Space Antenna (LISA). In addition, we demonstrate a method for measuring the circularity or eccentricity of AM CVn's accretion disc, using stroboscopic Doppler tomography. We test the predictions of an eccentric, precessing disc that are based on AM CVn's observed superhump behaviour. We limit the effective eccentricity in the outermost part of the disc, where the resonances that drive the eccentricity are thought to occur, to e = 0.04 +/- 0.01, which is smaller than previous models indicated.

The Thermal State of the Accreting White Dwarf in AM Canum Venaticorum Binaries

We calculate the heating and cooling of the accreting white dwarf (WD) in the ultracompact AM Canum Venaticorum (AM CVn) binaries and show that the WD can contribute significantly to their optical and ultraviolet emission. We estimate the WD's effective temperature, Teff, using the optical continuum for a number of observed binaries, and show that it agrees well with our theoretical calculations. Driven by gravitational radiation losses, the time averaged accretion rate, , decreases monotonically with increasing Porb, covering six orders of magnitude. If the short period (Porb 50,000$ K accreting WD. At longer Porb we calculate the Teff and absolute visual magnitude, M_V, that the accreting WD will have during low accretion states, and find that the WD naturally crosses the pulsational instability strip. Discovery and study of pulsations could allow for the measurement of the accumulated helium mass on the accreting WD, as well as its rotation rate. Accretion heats the WD core, but for Porb>40 minutes, the WD's Teff is set by its cooling as plummets. For the two long period AM CVn binaries with measured parallaxes, GP Com and CE 315, we show that the optical broadband colors and intensity are that expected from a pure helium atmosphere WD. This confirms that the WD brightness sets the minimum light in wide AM CVn binaries, allowing for meaningful constraints on their population density from deep optical searches, both in the field and in Globular Clusters.

XMM-Newtonobservations of AM CVn binaries

We present the results of XMM-Newton observations of four AM CVn systems – AM CVn, CR Boo, HP Lib and GP Com. Their light curves show very different characteristics. The X-ray light curves show no coherent pulsations, suggesting the accreting white dwarfs have relatively low magnetic field strengths. Their spectra were best modelled using a multi-temperature emission model and a strong UV component. We find that CR Boo and HP Lib have X-ray spectra with abundances consistent with relatively low temperature CNO processed material, while AM CVn and GP Com show an enhancement of nitrogen. A large fraction of the accretion luminosity is emitted in the UV. We determine accretion luminosities of ∼1.6 × 10 33 erg s −1 and 1.7 × 10 31 erg s −1 for AM CVn and GP Com respectively. Comparing the implied mass transfer rates with that derived using model fits to optical and UV spectra, we find evidence that in the case of AM CVn, we do not detect a significant proportion of the accretion energy. This missing component could be lost in the form of a wind.

SDSS J124058.03-015919.2: a new AM CVn star with a 37-min orbital period

We present high time-resolution spectroscopy with the Very Large Telescope of SDSS J124058.03−015919.2, a new helium-transferring binary star identified in the Sloan Digital Sky Survey. We measure an orbital period of 37.355 ± 0.002 min, confirming the AM CVn nature of the system. From the velocity amplitudes of the accretor and the accretion stream–disc impact, we derive a mass ratio q= 0.039 ± 0.010. Our spectral coverage extends from λλ3700–9500 A and shows the presence of helium, nitrogen, silicon and iron in the accretion disc, plus the redshifted, low-velocity 'central spikes' in the helium lines, known from the low-state AM CVn stars GP Com and CE 315. Doppler tomography of the helium and silicon emission lines reveals an unusual pattern of two bright emission sites in the tomograms, instead of the usual one emission site identified with the impact of the mass stream into the accretion disc. One of the two is preferred as the conventional stream–disc impact point in velocity space, at the 3σ confidence level. We speculate briefly on the origin of the second.

ChandraDetection of the AM Canum Venaticorum Binary ES Ceti (KUV 01584−0939)

We report on Chandra ACIS observations of the ultracompact AM CVn binary ES Cet. This object has a 10.3 minute binary period and is the most compact of the confirmed AM CVn systems. We have, for the first time, unambiguously detected the X-ray counterpart to ES Cet. In a 20 ks ACIS-S image a point-like X-ray source is found within 1'' of the cataloged optical position. The mean count rate in ACIS-S is 0.013 s-1, and there is no strong evidence for variability. We folded the X-ray data using the optical ephemeris of Warner & Woudt, but did not detect any significant modulation. If a ~100% modulation similar to those seen in the ultracompact candidates V407 Vul and RX J0806.3+1527 were present, then we would have detected it. The upper limit (3 σ) to any modulation at the putative orbital period is ~15% (rms). We extract the first X-ray spectrum from ES Cet, and find that it is not well described by simple continuum models. We find suggestive evidence for discrete spectral components at ~470 and 890 eV that can be modeled as Gaussian emission lines. In comparison with recent X-ray detections of nitrogen and neon in another AM CVn system (GP Com), it appears possible that these features may represent emission lines from these same elements; however, deeper spectroscopy will be required to confirm this. Our best spectral model includes a blackbody continuum with kT = 0.8 keV along with the Gaussian lines. The unabsorbed 0.2-5 keV X-ray flux was ~7 × 10-14 ergs cm-2 s-1. The observed luminosity in the hard component is a small fraction of the total expected accretion luminosity, most of which should be radiated below 0.1 keV at the expected mass accretion rate for this orbital period. We discuss the implications of our results for the nature of ES Cet.

Detection of Nitrogen and Neon in the X-Ray Spectrum of GP Comae Berenices with XMM/Newton

We report on X-ray spectroscopic observations with XMM/Newton of the ultracompact, double white dwarf binary GP Com. With the Reflection Grating Spectrometers (RGSs), we detect the Lyα and Lyβ lines of hydrogen-like nitrogen (N VII) and neon (Ne X), as well as the helium-like triplets (N VI and Ne IX) of these same elements. All the emission lines are unresolved. These are the first detections of X-ray emission lines from a double-degenerate, AM CVn system. We detect the resonance (r) and intercombination (i) lines of the N VI triplet, but not the forbidden (f) line. The implied line ratios for N VI, R = f/i < 0.3 and G = (f + i)/r ≈ 1, combined with the strong resonance line are consistent with formation in a dense, collision-dominated plasma. Both the RGS and EPIC/MOS spectra are well fitted by emission from an optically thin thermal plasma with an emission measure proportional to (kT/6.5 keV)0.8 (model cevmkl in XSPEC). Helium, nitrogen, oxygen, and neon are required to adequately model the spectrum; however, the inclusion of sulphur and iron further improves the fit, suggesting that these elements may also be present at low abundance. We confirm in the X-rays the underabundance of both carbon and oxygen relative to nitrogen, first deduced from optical spectroscopy by Marsh et al. The average X-ray luminosity of ≈3 × 1030 ergs s-1 implies a mass accretion rate ≈ 9 × 10-13 M☉ yr-1. The implied temperature and density of the emitting plasma, combined with the presence of narrow emission lines and the low- value, are consistent with production of the X-ray emission in an optically thin boundary layer just above the surface of the white dwarf.

New results on GP Com

We present high resolution optical and UV spectra of the 46 min orbital period, helium binary, GP Com. Our data contains simultaneous photometric correction which confirms the flaring behaviour observed in previous optical and UV data. In this system all lines show a triple peaked structure where the outer two peaks are associated with the accretion disc around the compact object. The main aim of this paper is to constrain the origin of the central peak, also called “central spike”. We find that the central spike contributes to the flare spectra indicating that its origin is probably the compact object. We also detect that the central spike moves with orbital phase following an S-wave pattern. The radial velocity semiamplitude of the S-wave is ~10 km s-1 indicating that its origin is near the centre of mass of the system, which in this case lies very close to the white dwarf. Our resolution is higher than that of previous data which allows us to resolve structure in the central peak of the line. The central spike in three of the He i lines shows another peak blueshifted with respect to the main peak. We propose that one of the peaks is a neutral helium forbidden transition excited in a high electron density region. This forbidden transition is associated with the permitted one (the stronger peak in two of the lines). The presence of a high electron density region again favours the white dwarf as their origin. We compute Doppler maps for the emission lines which show three emission regions: an accretion disc, a bright spot and an unidentified low velocity emission region associated with the central spike. We obtain modulation Doppler tomograms for some of the emission lines that map the anisotropic emission from the bright spot region. The HST UV spectra also show a strong flare component and confirm the under abundance of silicon in GP Com.

High-speed photometry of faint cataclysmic variables - I. V359 Cen, XZ Eri, HY Lup, V351 Pup, V630 Sgr, YY Tel, CQ Vel and CE-315

The first results of a photometric survey of faint Cataclysmic Variables are presented. V359 Cen is an SU UMa star with a period of 112 min. Even though observed at quiescence, the mass transfer rate in this old nova may be sufficiently high that in such a short period system (with its implied small mass ratio) the disc may be excited into an elliptical shape with the result that the observed brightness modulation gives a superhump period rather than an orbital period. XZ Eri is an eclipsing dwarf nova with an orbital period (P(orb)) of 88.1 min. HY Lup has only slight variability. V351 Pup, the remnant of Nova Puppis 1991, has P(orb) = 2.837 h and a light curve that strongly resembles that of the magnetic Nova Cyg 1975. V630 Sgr is the first nova remnant that has both positive superhumps (P(sh) = 2.980 h) and eclipses (P(orb) = 2.831 h). The YY Tel identification is somewhat uncertain. The correct identification for CQ Vel is provided from discovery of its flickering activity. The light curve of CE-315, a recently discovered AM CVn star, shows similarities to that of GP Com, with no apparent orbital modulation.

The Quiescent Spectrum of the AM Canum Venaticorum Star CP Eridani

We used the 6.5 m Multiple Mirror Telescope to obtain a spectrum of the AM Canum Venaticorum star CP Eridani in quiescence. The spectrum is dominated by He I emission lines, which are clearly double-peaked with a peak-to-peak separation of ~1900 km s-1. The spectrum is similar to that of the longer period AM CVn systems GP Comae Berenices and CE 315, linking the short- and long-period AM CVn systems. In contrast to GP Com and CE 315, the spectrum of CP Eri does not show a central spike in the line profiles, but it does show lines of Si II in emission. The presence of these lines indicates that the material being transferred is of higher metallicity than in GP Com and CE 315, which, combined with the low proper motion of the system, probably excludes a halo origin of the progenitor of CP Eri. We constrain the primary mass to M1 > 0.27 M☉ and the orbital inclination to 33° < i 80°. The presence of the He I lines in emission opens up the possibility for phase-resolved spectroscopic studies, which will allow us to determine the system parameters and to study in detail helium accretion disks under highly varying circumstances.

Infrared spectroscopy of cataclysmic variables -- III. Dwarf novae below the period gap and nova-like variables

We present K-band spectra of the short-period dwarf novae YZ Cnc, LY Hya, BK Lyn, T Leo, SW UMa and WZ Sge, the nova-like variables DW UMa, V1315 Aql, RW Tri, VY Scl, UU Aqr and GP Com, and a series of field dwarf stars with spectral types ranging from K2 to M6. The spectra of the dwarf novae are dominated by emission lines of H i and He i. The large velocity and equivalent widths of these lines, in conjunction with the fact that the lines are double-peaked in the highest inclination systems, indicate an accretion disc origin. In the case of YZ Cnc and T Leo, for which we obtained time-resolved data covering a complete orbital cycle, the emission lines show modulations in their equivalent widths that are most probably associated with the bright spot (the region where the gas stream collides with the accretion disc). There are no clear detections of the secondary star in any of the dwarf novae below the period gap, yielding upper limits of 10–30 per cent for the contribution of the secondary star to the observed K-band flux. In conjunction with the K-band magnitudes of the dwarf novae, we use the derived secondary star contributions to calculate lower limits to the distances to these systems. The spectra of the nova-like variables are dominated by broad, single-peaked emission lines of H i and He i– even the eclipsing systems we observed do not show the double-peaked profiles predicted by standard accretion disc theory. With the exception of RW Tri, which exhibits Na i, Ca i and 12CO absorption features consistent with a M0V secondary contributing 65 per cent of the observed K-band flux, we find no evidence for the secondary star in any of the nova-like variables. The implications of this result are discussed.

Kinematics of the helium accretor GP Com

I present and analyse time-resolved B-band spectra of the double-degenerate binary star GP Com. The spectra confirm the presence and period (46.5 min) of the `S'-wave feature found by Nather, Robinson & Stover. GP Com is erratically variable at X-ray and UV wavelengths. I find the equivalent variability in my data, which, as also seen in UV data, is mostly confined to the emission lines. The He ii 4686 changes by the largest amount, consistent with X-ray-driven photoionization. The flaring part of the line profiles is broader than the average, as expected if they are dominated by the inner disc. The He ii 4686 profile is especially remarkable in that its blueshifted peak is 1400 km s-1 from line centre compared with 700 km s-1 for the He i lines (the redshifted peak is blended with He i 4713). I deduce that He ii 4686 emission is confined to the inner 1/4 of the disc. I also suggest that the activity of the inner disc indicates that accretion is significant (and unstable) there, in contrast to quiescent dwarf novae; this supports models in which GP Com is in a (quasi-)steady state of low mass transfer rate. GP Com shows triple-peaked line profiles, which consist of the usual double-peaked profiles from a disc plus a narrow component at line centre. The latter has previously been ascribed to emission from a nebula, although none could be found in direct images. However, I find evidence for both radial velocity and flux variability in this component, inconsistent with a nebula origin. The radial velocity amplitude of 10.8±1.6 km s−1 and its phase relative to the `S'-wave are consistent with an origin on the accreting white dwarf, if the mass ratio, q=M2/M1, is of order 0.02, as expected on evolutionary grounds. However, this explanation is still not satisfactory, as the systemic velocity of the narrow component shows significant variation from line to line, and I have no explanation for this.