The long-period AM CVn star SDSS J155252.48+32 0150.9

The Sloan Digital Sky Survey has been instrumental in obtaining a homogeneous sample of the rare AM CVn stars: mass-transferring binary white dwarfs. As part of a campaign of spectroscopic follow-up on candidate AM CVn stars from the Sloan Digital Sky Survey, we have obtained time-resolved spectra of the   g = 20.2  candidate SDSS J155252.48+320150.9 on the Very Large Telescope of the European Southern Observatory. We report an orbital period   P _orb= 3376.3 ± 0.3 s  , or 56.272 ± 0.005 min, based on an observed 'S-wave' in the helium emission lines of the spectra. This confirms the ultracompact nature of the binary. Despite its relative closeness to the orbital period minimum for hydrogen-rich donors, there is no evidence for hydrogen in the spectra. We thus classify SDSS J1552 as a new bona fide AM CVn star, with the second-longest orbital period after V396 Hya  ( P = 65.5 min)  . The continuum of SDSS J1552 is compatible with either a blackbody or helium atmosphere of   T _eff= 12 000–15 000 K  . If this represents the photosphere of the accreting white dwarf, as is expected, it puts the accretor at the upper end of the temperature range predicted by thermal evolution models. This suggests that SDSS J1552 consists of (or formerly consisted of) relatively high-mass components.     

Spectroscopic evidence for two-pole accretion in the intermediate polar RX J0558+5353

We report on time-series optical spectrophotometry of the low-inclination intermediate polar RX  J0558+5353. This object exhibits coherent continuum and Hα line variations on both the orbital and white dwarf spin cycles. Despite the absence of a well-determined conjunction phase, the spectroscopic ephemeris combined with a favourable comparison with better-studied systems suggests that the impact between gas stream and accretion disc, or magnetosphere, drives orbital phenomena. Continuum variations over the spin cycle are consistent with previous broad-band data and indicate that accretion occurs on two poles of the primary star. We find no significant spin pulsations in the integrated line emission, but do detect variations in the resolved line profiles of Hα. Pulsed profiles are also suggestive of two-pole accretion. Double-peaked Hα line profiles and the non-detection of beat behaviour between the orbital and spin frequencies provide evidence for partial-disc transport in the system.     

Observations of the SW Sextantis star UU Aquarii

We present 14 nights of medium resolution (1–2 Å) spectroscopy of the eclipsing cataclysmic variable UU Aquarii obtained during a high accretion state in 1995 August–October. UU Aqr appears to be an SW Sextantis (SW Sex) star, as noted by Baptista, Steiner & Horne, and we discuss its spectroscopic behaviour in the context of the SW Sex phenomenon. Emission-line equivalent width curves, Doppler tomography, and line profile simulation provide evidence for the presence of a bright spot at the impact site of the accretion stream with the edge of the disc, and a non-axisymmetric, vertically and azimuthally extended absorbing structure in the disc. The absorption has maximum depth in the emission lines around orbital phase 0.8, but is present from φ≈0.4 to φ≈0.95. An origin is explored for this absorbing structure (as well as for the other spectroscopic behaviour of UU Aqr) in terms of the explosive impact of the accretion stream with the disc.     

X-ray confirmation of the new intermediate polar RX J1238–38

By finding a 2147-s X-ray pulsation in the recently identified ROSAT   source RX  1238–38 we confirm that it is a member of the intermediate polar class of cataclysmic variable. We analyse the spectral changes over the white dwarf spin cycle, but are unable to distinguish between competing mechanisms for the cause of the pulsation. RX  1238–38 has an anomalous ratio of spin period to orbital period, similar to that of EX Hya.     

SDSS J080449.49+161624.8: a peculiar AM CVn star from a colour-selected sample of candidates

We describe a spectroscopic survey designed to uncover an estimated ∼40 AM Canum Venaticorum (AM CVn) stars hiding in the photometric data base of the Sloan Digital Sky Survey. We have constructed a relatively small sample of about 1500 candidates based on a colour selection, which should contain the majority of all AM CVn binaries while remaining small enough that spectroscopic identification of the full sample is feasible.
We present the first new AM CVn star discovered using this strategy, SDSS J080449.49+161624.8, the ultracompact binary nature of which is demonstrated using high-time-resolution spectroscopy obtained with the Magellan telescopes at Las Campanas Observatory, Chile. A kinematic 'S-wave' feature is observed on a period   P _orb= 44.5 ± 0.1 min  , which we propose is the orbital period, although the present data cannot yet exclude its nearest daily aliases.
The new AM CVn star shows a peculiar spectrum of broad, single-peaked helium emission lines with unusually strong series of ionized helium, reminiscent of the (intermediate) polars among the hydrogen-rich cataclysmic variables. We speculate that SDSS J0804+1616 may be the first magnetic AM CVn star. The accreted material appears to be enriched in nitrogen, to N/O ≳ 10 and N/C > 10 by number, indicating CNO cycle hydrogen burning, but no helium burning, in the prior evolution of the donor star.     

V348 Puppis: a new SW Sex star in the period gap

We present time-resolved optical spectroscopy and photometry of the nova-like cataclysmic variable V348 Puppis. The system displays the same spectroscopic behaviour as SW Sex stars, so we classify V348 Pup as a new member of the class. V348 Pup is the second SW Sex system (the first is V795 Herculis) which lies in the period gap. The spectra exhibit enhanced He  ii λ 4686 emission, reminiscent of magnetic cataclysmic variables. The study of this emission line gives a primary velocity semi-amplitude of     . We have also derived the system parameters, obtaining:     ,         ,     and     . The spectroscopic behaviour of V348 Pup is very similar to that of V795 Her, with the exception that V348 Pup shows deep eclipses. We have computed the '0.5-absorption' spectrum of both systems, obtaining spectra that resemble the absorption spectrum of a B0 V star. We propose that absorption in SW Sex systems can be produced by a vertically extended atmosphere which forms where the gas stream re-impacts the system, either at the accretion disc or at the magnetosphere of the white dwarf (assuming a magnetic scenario).     

Discovery of the high-field polar RX J1724.0+4114

We report the discovery of a new AM Herculis binary (polar) as the optical counterpart of the soft X-ray source RX J1724.0+4114 detected during the ROSAT all-sky survey. The magnetic nature of this V  ∼ 17 mag object is confirmed by low-resolution spectroscopy showing strong Balmer and He  II emission lines superimposed on a blue continuum, which is deeply modulated by cyclotron humps. The inferred magnetic field strength is 50 ± 4 MG (or possibly even ≈ 70 MG). Photometric observations spanning ∼ 3 yr reveal a period of 119.9 min, directly below the period gap. The morphology of the optical and X-ray light curves, which do not show eclipses by the secondary star, suggests a self-eclipsing geometry. We derive a lower limit on the distance of d  ≳ 250 pc.     

SDSS J233325.92+152222.1 and the evolution of intermediate polars

Intermediate polars (IPs) are cataclysmic variables which contain magnetic white dwarfs with a rotational period shorter than the binary orbital period. Evolutionary theory predicts that IPs with long orbital periods evolve through the 2–3 h period gap, but it is very uncertain what the properties of the resulting objects are. Whilst a relatively large number of long-period IPs are known, very few of these have short orbital periods. We present phase-resolved spectroscopy and photometry of SDSS J233325.92+152222.1 (SDSS J2333) and classify it as the IP with the shortest-known orbital period (83.12 ± 0.09 min), which contains a white dwarf with a relatively long spin period (41.66 ± 0.13 min). We estimate the white dwarf's magnetic moment to be μ_WD≈ 2 × 10³³ G cm³, which is not only similar to three of the other four confirmed short-period IPs but also to those of many of the long-period IPs. We suggest that long-period IPs conserve their magnetic moment as they evolve towards shorter orbital periods. Therefore, the dominant population of long-period IPs, which have white dwarf spin periods roughly 10 times shorter than their orbital periods, will likely end up as short-period IPs like SDSS J2333, with spin periods a large fraction of their orbital periods.     

The spin period of EX Hydrae

We show that the spin period of the white dwarf in the magnetic cataclysmic variable (CV) EX Hydrae represents an equilibrium state in which the corotation radius is comparable with the distance from the white dwarf to the inner Lagrange point. We also show that a continuum of spin equilibria exists at which P _spin is significantly longer than ∼0.1 P _orb. Most systems occupying these equilibrium states should have orbital periods below the CV period gap, as observed.     

Periodicities in the high-mass X-ray binary system RX J0146.9+6121/LS I+61°235

The high-mass X-ray binary RX J0146.9+6121, with optical counterpart LS I+61°235 (V831 Cas), is an intriguing system on the outskirts of the open cluster NGC 663. It contains the slowest Be type X-ray pulsar known with a pulse period of around 1400 s and, primarily from the study of variation in the emission line profile of Hα, it is known to have a Be decretion disc with a one-armed density wave period of approximately 1240 d. Here we present the results of an extensive photometric campaign, supplemented with optical spectroscopy, aimed at measuring short time-scale periodicities. We find three significant periodicities in the photometric data at, in order of statistical significance, 0.34, 0.67 and 0.10 d. We give arguments to support the interpretation that the 0.34 and 0.10 d periods could be due to stellar oscillations of the B-type primary star and that the 0.67 d period is the spin period of the Be star with a spin axis inclination of  23⁺¹⁰₋₈  degrees. We measured a systemic velocity of  −37.0 ± 4.3 km s⁻¹  confirming that LS I+61°235 has a high probability of membership in the young cluster NGC 663 from which the system's age can be estimated as 20–25 Myr. From archival RXTE All Sky Monitor (ASM) data we further find 'super' X-ray outbursts roughly every 450 d. If these super outbursts are caused by the alignment of the compact star with the one-armed decretion disc enhancement, then the orbital period is approximately 330 d.     

V533 Herculis: the second SW Sex old nova displaying emission-line flaring

We present high-time-resolution spectroscopy of the non-eclipsing old nova V533 Herculis (N Her 1963). It is the second nova remnant affected by the 'SW Sex syndrome'. A modulation of the equivalent width of the emission lines with a period of 23.33 min has been detected. This, together with the strong He ii λ4686 emission characteristic of magnetic systems, leads us to link this period to the spin of a magnetic white dwarf. Similar flaring activity has been recorded in other SW Sex stars, namely, the old nova BT Mon, LS Peg and DW UMa, supporting the idea of these systems being magnetic accretors. Stationary emission features are also observed in the Balmer lines, which we attribute to the ejected nova shell.