Hot white dwarfs in detached binaries from the Rosat WFC All Sky Survey

White dwarfs in unresolved pairs with normal stars (spectral type K or earlier) are invisible at optical wavelengths, due to the close proximity of the much more luminous main sequence companion. ROSAT has provided evidence for the existence of a growing sample of these hidden white dwarfs through the detection of EUV and soft X-ray emission. For companions of spectral type ~A5 or earlier, the white dwarf can be spectroscopically identified at far-ultraviolet wavelengths by IUE. Eleven such systems had previously been found in this way from ROSAT, EUVE and IUE observations. A search for fainter, less obvious samples of these binaries is presented, and five new systems have been discovered.;Three new close, pre-CV WD+dM binaries have also been found in the ROSAT WFC survey. Intriguingly, all three degenerates are rare mixed hydrogen/helium atmosphere DAO white dwarfs. The EUVE spectrum of one of these new systems, RE J0720-318, is analysed in detail. In particular, it is found that, while the optical spectrum can only be reproduced with a homogeneously mixed atmosphere, the EUVE spectrum can only be matched by a layered model, implying that the underlying structure of the white dwarf is stratified. The hydrogen layer mass of 3x10-14 M is the lowest measured for any white dwarf from EUVE spectra. In addition, an unprecedented HeI/HI ratio of ~1 is detected for the absorbing column along the line of sight, implying a hydrogen ionisation fraction of >90%, if all of this material resides in local interstellar medium. It is suggested that most of the helium lies in the vicinity of the star, possible in the form of a circumbinary disk left over from the common envelope phase. These results have important implications for our understanding of the evolutionary status of DAO white dwarfs in particular, and for post-common envelope systems in general.;A catalogue of all the detected white dwarf binaries found in the ROSAT survey is presented, with an analysis of the white dwarf mass distribution. Compared with optically selected samples, a significant excess of hot, massive objects is detected. This excess probably arises from the slower cooling rates of massive (>0.9M) white dwarfs in comparison to normal mass (0.6M) stars.