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We have identified photometric white dwarf candidates in the WFCAM transit survey through a reduced proper motion versus colour approach. Box-fitting with parameters adjusted to detect the unique signature of a white dwarf + planet/brown dwarf transit/eclipse event was performed, as well as looking for variability due to the irradiation of the companions atmosphere by the white dwarf's high UV flux. We have also performed a simple sensitivity analysis in order to assess the ability of the survey to detect companions to white dwarfs via the transit method.

WD 1953–011 is an isolated, coolmagnetic white dwarf (MWD) with a low average field strength (∼70 kG), and a higher than average mass. Spectroscopic observations taken by Maxted et al. showed variations of equivalent width in the Balmer lines, unusual in a low-field white dwarf. Here we presentV-band photometry of WD 1953–011 taken at seven epochs over a total of 22 months. All of the data sets show a sinusoidal variation of approximately 2 per cent peak-to-peak amplitude. We propose that these variations are due to a star-spot on the MWD, analogous to a sunspot, which is affecting the temperature at the surface, and therefore its photometric magnitude. The variations have a best-fitting period over the entire 22 months of 1.4418 d, which we interpret as the rotational period of the white dwarf. [ABSTRACT FROM AUTHOR]

ABSTRACT The observation of the strengths and profiles of the hydrogen Balmer absorption series is an established technique for determining the effective temperature and surface gravity of hot H-rich white dwarf stars. In principle, the Lyman series lines should be equally useful but, lying in the far-ultraviolet (FUV), are only accessible from space. Nevertheless, there are situations (for example, where the optical white dwarf spectrum is highly contaminated by the presence of a companion) in which use of the Lyman series may be essential. Therefore, it is important to establish whether or not the Lyman lines provide an equally valid means of measurement. We have already made a first attempt to study this problem, comparing Lyman line measurements from a variety of FUV instruments with ground-based Balmer line studies. Within the measurement uncertainties, we found the results from each line series to be broadly in agreement. However, we noted a number of potential systematic effects that could bias either measurement. With the availability of the Far Ultraviolet Spectroscopic Explorer (FUSE ) data archive and observations from our own Guest Observer programmes, we now have an opportunity to examine the use of the Lyman series in more detail from observations of 16 DA white dwarfs. Here we have data produced by a single instrument and processed with a uniform data reduction pipeline, eliminating some of the possible systematic differences between observations of the same or different stars. We have also examined the scatter in values derived from multiple observations of the same star, which is significant. The new results partially reproduce the earlier study, showing that Balmer and Lyman line determined temperatures are in good agreement up to ∼50 000 K. However, above this value there is an increasing systematic difference between the Lyman and Balmer line results, the former yielding the higher temperature. At the moment, there is no clear explanation of this effect but we think that it is most likely associated with deficiencies in the detailed physics incorporated into the stellar model atmosphere calculations. Even so, the data do demonstrate that, for temperatures below 50 000 K, the Lyman lines give reliable results. Furthermore, for the hotter stars, a useful empirical calibration of the relationship between the Lyman and Balmer measurements has been obtained, which can be applied to other FUSE observations. [ABSTRACT FROM AUTHOR]

We present spectra of the DAB white dwarf PG 1115+166. Radial velocity measurements of the Balmer lines and the He i 6678 Å line show that this is a binary white dwarf with a period of 30.09 d in which the Balmer lines move in anti-phase to the He i line, i.e. PG 1115+166 is a DA+DB binary. The minimum masses of the stars are M[sub DA]= 0.43 ± 0.15 M[sub ⊙] and M[sub DB]= 0.52 ± 0.12 M[sub ⊙]. The separation of the stars is about 45 R[sub ⊙], which is much smaller than a typical asymptotic giant branch (AGB) progenitor of a white dwarf, implying that there has been at least one common envelope phase in this binary. Indeed, it is possible that this binary may have suffered up to three mass transfer episodes – two associated with the red giant phase prior to the formation of each white dwarf, and a third associated with the ‘born-again’ red giant phase of the DB white dwarf. PG 1115+166 has the longest orbital period of any post common envelope white dwarf–white dwarf binary found to date. Published models for the formation of white dwarf–white dwarf binaries do not predict any white dwarfs with the combination of a long orbital period and high mass found in PG 1115+166. We conclude that PG 1115+166 is a key object for testing models of binary star evolution, and it may also be a key object for our understanding of the formation of DB white dwarfs. We outline the observational tests which can be applied to scenarios for the formation of PG 1115+166 and apply them to the simplest case of a single common-envelope phase. This suggests that some part of the internal energy stored in the envelope of the AGB star, e.g. as ionized hydrogen, may have contributed to the ejection of the common envelope, but there are several unanswered questions concerning this simple scenario. [ABSTRACT FROM AUTHOR]

Results are presented from a multisite photometric campaign on the pulsating subdwarf B star, PG 1047+003. Approximately 98 h of data were obtained from three sites, with a duty cycle of ∼30 per cent. The light curve of PG 1047+003 is dominated by several frequencies in the range ∼6880–7500 μHz (periods 133–145 s). We identify, with some confidence, 18 frequencies between 5700 and 9605 μHz (104–175 s) down to an amplitude of 0.0004 in fractional intensity (0.04 per cent), thus doubling the number of published pulsation frequencies for this star. These pulsations correspond to low-order and low-degree acoustic modes. Several frequency spacings near 1.05 μHz suggest a rotation period for the star of 11.0 d. [ABSTRACT FROM AUTHOR]

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the very hot (T[sub eff]≈60 000 K) DA white dwarf PG1342+444, describing our data reduction and analysis techniques. The spectrum reveals a number of photospheric absorption lines from high ionization species along with numerous interstellar features. The photospheric detections include the 1031.9- and 1037.0-Å O vi lines which are seen for the first time in a hot DA atmosphere and are usually associated with the much hotter PG1159 stars and so-called O vi central stars of planetary nebulae. Estimates of the stellar effective temperature made independently using both the Balmer and Lyman series lines are in disagreement (T[sub eff]≈67 000 and ≈54 000 K respectively), when taking into account just the statistical uncertainties in the analyses. However, the presence of weak absorption from the C iii multiplet near 1176 Å, which is predicted to be much stronger if the star were as cool as the Lyman measurement suggests, leads us to favour the higher temperature. PG1342+444 appears to have enhanced C, Fe and Ni abundances in its atmosphere compared with all the other G191-B2B-like DA white dwarfs, which might affect the temperature structure of the atmosphere if not homogeneously distributed, as assumed in this study. [ABSTRACT FROM AUTHOR]

We present FUSE H Lyman series spectroscopy of the hot white dwarf companion to the 4th magnitude A1 III star β Crt, which shows that it has an unusually low mass, , and has almost certainly evolved through binary interaction. This system could be a long-sought remnant of Algol-type evolution, although radial velocity measurements appear to show that the pair are not close. Instead, micro-variations in the proper motion of β Crt as measured by Hipparcos suggest that the period could be as high as ∼10 yr. However, a low-mass white dwarf in a system with a period ≳3 yr is difficult to explain by conventional models for binary evolution. We speculate on alternative models for the evolution of this system which involve an eccentric binary or multiple components. [ABSTRACT FROM AUTHOR]

During re-processing and analysis of the entire ROSAT Wide Field Camera (WFC) pointed observations data base, we discovered a serendipitous, off-axis detection of the cataclysmic variable SW UMa at the onset of its 1997 October superoutburst. Although long outbursts in this SU UMa-type system are known to occur every ∼ 450 d, none had ever been previously observed in the extreme ultra-violet (EUV) by ROSAT. The WFC observations began just ≈13 hr after the optical rise was detected. With a peak count rate of ∼ 4.5 count s[sup -1] in the S1 filter, SW UMa was temporarily the third brightest object in the sky in this waveband. Over the next ≈19 hr the measured EUV flux dropped to < 2 count s[sup -1], while the optical brightness remained essentially static at m[sub v]∼11. Similar behaviour has also been recently reported in the EUV light curve of the related SU UMa-type binary OY Car during superoutburst, as reported by Mauche & Raymond. In contrast, U Gem-type dwarf novae show no such early EUV dip during normal outbursts. Therefore, this feature may be common in superoutbursts of SU UMa-like systems. We expand on ideas first put forward by Osaki and Mauche & Raymond and offer an explanation for this behaviour by examining the interplay between the thermal and tidal instabilities that affect the accretion discs in these systems. [ABSTRACT FROM AUTHOR]

We present initial results from a Hubble Space Telescope ultraviolet imaging survey of stars known to have hot white dwarf companions which are unresolved from the ground. The hot companions, discovered through their EUV or UV emission, are hidden by the overwhelming brightnesses of the primary stars at visible wavelengths. Out of 17 targets observed, we have resolved eight of them with the Wide Field Planetary Camera 2, using various ultraviolet filters. Most of the implied orbital periods for the resolved systems are hundreds to thousands of years, but in at least three cases (56 Persei, ζ Cygni and RE J1925-566) it should be possible to detect the orbital motions within the next few years, and they may eventually yield new dynamically determined masses for the white dwarf components. The 56 Persei and 14 Aurigae systems are found to be quadruple and quintuple, respectively, including the known optical components as well as the newly resolved white dwarf companions. The mild barium star ζ Cygni, known to have an 18-year spectroscopic period, is marginally resolved. All of these newly resolved Sirius-type binaries will be useful in determining gravitational redshifts and masses of the white dwarf components. [ABSTRACT FROM AUTHOR]

Multiwavelength monitoring observations of the AM Her system, QS Tel, are presented, the centrepiece being a coordinated campaign with the ASCA, EUVE and HST satellites and optical observations on 1996 September 28–29. The star was initially observed in an intermediate brightness, single pole state in 1996 April by ROSAT. However two EUVE snapshots of the system in August found it in a deep low state, the light curves appearing to retain a bright and faint phase morphology although the bright interval showed significant phase drifts compared with the earlier ROSAT data. At this time, the system appeared optically bright, while polarimetry pointed to one active pole. A modest recovery of intensity, albeit accompanied by an apparently chaotic extreme ultraviolet (EUV) light curve, was subsequently recorded during the coordinated run in late September. This brightening was confirmed by a second ROSAT pointing in early October, although the EUV brightening is matched by a marked decay in its optical output. These observations, all of which found the system in a fainter EUV state than seen in 1993, add to the catalogue of accretion mode behaviour in QS Tel. However, as yet, the data base of observations does not allow us to establish whether the accretion mode is dictated by accretion-rate dependent penetration of the magnetosphere or asynchronous rotation. The ASCA spectrum obtained during the coordinated observation is adequately described by a thermal spectrum with a temperature of about 4.4 keV and an absorbing column density of less than 8×10[sup 20] cm[sup -2]. At the same time, the 1150–2600 Å UV HST spectrum shows a strong, broad Lyα absorption profile which we attribute to the white dwarf. We infer an effective temperature of 16 000–19 000 K and a mass between 0.5 and 0.93 M[sub ⊙] for the white dwarf, at least some of the uncertainty arising because the profile is very probably contaminated and broadened by Zeeman features. The orbital variation seen by HST can be matched by a blackbody-like spectrum with a temperature above 24 000 K. The near UV part of the spectrum contains evidence of phase-variable cyclotron harmonics. [ABSTRACT FROM AUTHOR]

We present a previously unpublished ROSAT Wide Field Camera observation of the transient source RE J1255+266 made just 4 d before the discovery observations. The source is not detected, limiting the duration of the outburst to be less than expected for a superoutburst of a WZ Sge system. We also present a marginal detection of X-ray emission from RE J1255+266 using ASCA. The most probable luminosity is 6×10[sup 29] erg s[sup -1], which is very similar to WZ Sge itself. We discuss the nature of the source in the light of these observations, and conclude that it is most probably a WZ Sge system, but that the observed outburst must have been a normal dwarf nova outburst. [ABSTRACT FROM AUTHOR]

GD 356 is a magnetic white dwarf that uniquely displays weak resolved Zeeman triplets of Hα and Hβ in emission. As such, GD 356 may be the only known white dwarf with some kind of chromosphere, although accretion from the interstellar medium or more exotic mechanisms cannot be ruled out. Here, we report the detection of low amplitude (±∼0.2 per cent) near-sinusoidal photometric ( V-band) variability in GD 356, with a period of 0.0803 d (∼115 min). We interpret this as the rotation period of the star. We model the variability with a dark spot (by analogy with star spots) covering 10 per cent of the stellar surface. It seems likely that this spot is also the site of the Zeeman emission, requiring the presence of a temperature inversion. We show that the spot is never totally visible or obscured, and that both polar and equatorial spots produce good fits to the data when viewed at high and low inclination respectively. [ABSTRACT FROM AUTHOR]