A search for hidden white dwarfs in the ROSAT EUV survey - II. Discovery of a distant DA+F6/7V binary system in a direction of low-density neutral hydrogen

The ROSAT Wide Field Camera (WFC) survey of the Extreme Ultraviolet (EUV) has provided us with evidence for the existence of a previously unidentified sample of hot white dwarfs in unresolved, detached binary systems. These stars are invisible at optical wavelengths due to the close proximity of their much more luminous compan- ions (spectral type K or earlier). However, for companions of spectral typeA5 or later the white dwarfs are easily visible at far-ultraviolet (far-UV) wavelengths, and can be identified in spectra taken by IUE. Sixteen such systems have been discovered in this way through ROSAT, EUVE and IUE observations, including four identified by us in Paper I (Burleigh, Barstow and Fleming 1997). In this paper we report the results of our continuing search during the final year of IUE operations. One new sys- tem, RE J0500 364 (DA+F6/7V), has been identified. This star appears to lie at a distance of between �500 1000pc, making it one of the most distant white dwarfs, if not the most distant, to be detected in the EUV surveys. The very low line-of-sight neutral hydrogen volume density to this object could place a lower limit on the length of theCMa interstellar tunnel of diffuse gas, which stretches away from the Local Bubble in a similar direction to RE J0500 364. In this paper we also analyse a number of the stars observed where no white dwarf companion was found. Some of these objects show evidence for chromospheric and coronal activity. Finally, we present an analysis of the previously known WD+active F6V binary HD27483 (Bohm-Vitense 1993), and show that, at T�22,000K, the white dwarf may be contributing significantly to the observed EUV flux. If so, it is one of the coolest such stars to be detected in the EUV surveys.