DETECTION OF A NEW WHITE-DWARF BINARY-SYSTEM IN THE EXTREME ULTRAVIOLET USING THE ROSAT WIDE FIELD CAMERA

DETAILED understanding of the evolution of main-sequence stars into white dwarfs depends on knowledge of the chemical composition of white-dwarf atmospheres, but the hottest white dwarfs emit much of their radiation in the extreme ultraviolet (EUV), a part of the spectrum that has been generally inaccessible to astronomers. We report here the discovery of a new white dwarf as a bright EUV source found during the first observations using the Wide Field Camera 1 on the Rosat satellite. Within the 1-arcmin error box of the EUV source is an unusually blue star, which we show to be a binary system consisting of a DA white dwarf and a cool companion in the classification range dM2-dM5. There are some similarities between this new object and the binary system Feige 24 (ref. 2), but spectral differences in the EUV emission can be attributed to significantly different atmospheric compositions for the two white dwarfs.