A search for hidden white dwarfs in the ROSAT extreme ultraviolet survey

The ROSAT Wide Field Camera survey has provided us with evidence for the existence of a previously unidentified sample of hot white dwarfs (WDs) in noninteracting binary systems, through the detection of extreme ultraviolet (EUV) and soft X-ray emission. These stars are hidden at optical wavelengths because of their close proximity to much more luminous main-sequence (MS) companions (spectral type K or earlier). However, for companions of spectral type similar to A5 or later, the white dwarfs are easily visible at far-UV wavelengths, and can be identified in spectra taken by the International Ultraviolet Explorer (IUE). Eleven WD binary systems have previously been found in this way from ROSAT, Extreme Ultraviolet Explorer (EUVE) and IUE observations. In this paper we report the discovery of three more such systems through our programmes in recent episodes of IUE. The new binaries are HD 2133, RE J0357 + 283 (the existence of which was predicted by Jeffries, Burleigh & Robb in 1996), and BD + 27 degrees 1888. In addition, we have independently identified a fourth new WD + MS binary, RE J1027 + 322, which has also been reported in the literature by Geneva et al., bringing the total number of such systems discovered as a result of the EUV surveys to 15. We also discuss here six stars which were observed as part of the programme, but for which no white dwarf companion was found. Four of these are coronally active. Finally, we present an analysis of the WD + K0IV binary HD 18131, which includes the ROSAT PSPC X-ray data.