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-DENSITYNEUTRAL HYDROGEN/

The ROSAT Wide Field Camera (WFC) survey of the extreme ultraviolet EU V) has provided us with evidence for the existence of a previously uni dentified sample of hot white dwarfs in unresolved, detached binary sy stems. These stars are invisible at optical wavelengths due to the clo se proximity of their much more luminous companions (spectral type K o r earlier). However, for companions of spectral type similar to A5 or later the white dwarfs are easily visible at far-ultraviolet wavelengt hs, and can be identified in spectra taken by IUE. 16 such systems hav e been discovered in this way through ROSAT, EWE and IUE observations, including four identified by us in Paper I. In the present paper we r eport the results of our continuing search during the final year of IU E operations. One new system, RE J0500-364 (DA+F6/7V), has been identi fied. This star appears to lie at a distance of similar to 500-1000 pc , making it one of the most distant white dwarfs, if not the most dist ant, to be detected in the EUV surveys. The very low line-of-sight neu tral hydrogen volume density to this object could place a lower limit on the length of the beta CMa interstellar tunnel of diffuse gas, whic h stretches away from the Local Bubble in a similar direction to RE J0 500-364. In this paper we also analyse a number of the stars observed where no white dwarf companion was found. Some of these objects show e vidence for chromospheric and coronal activity. Finally, we present an analysis of the previously known WD+active F6V binary HD 27483 (Bohm- Vitense 1993), and show that, at T approximate to 22000K, the white dw arf 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 .