THE NATURE OF THE NEWLY DISCOVERED DO WHITE-DWARF RE-0503-289

We report the discovery of a hot DO white dwarf, RE 0503 - 289, which lies in a region of very low interstellar H I density. The star was found independently by the ROSAT EUV and Edinburgh-Cape surveys. It lies within 1-degrees of the hot DA white dwarf RE 0457 - 281 which we have reported as having the lowest directly measured H I column. A follow-up observation made using the Voyager 2 ultraviolet spectrometer reveals a strong continuum shortward of the 912-angstrom Lyman limit, and hence a similarly low hydrogen column density of 1.3 x 10(17) atom cm-2. Optical and UV spectroscopy gives a temperature in the range 60000-80000 K and a carbon abundance almost-equal-to 1 per cent. The observed EUV fluxes, however, are significantly lower than predicted by this temperature and composition, indicating that trace elements not evident in the optical spectrum are contributing to the EUV opacity. RE 0503 - 289 is the only DO white dwarf detected in the ROSAT sky survey. Since the interstellar opacity is extremely low, it represents a unique opportunity to search for the presence of trace heavy elements with EUV spectroscopy and to learn more about the composition of DO stars.