RE-1738- THE HOTTEST DA WHITE-DWARF DETECTED BY ROSAT(665 )

One of the most important results of the ROSAT all-sky X-ray and EUV s urveys has been the discovery of a large number of previously uncatalo gued white dwarfs. One of these, RE 1738 + 665, is found to be the hot test hydrogen-rich DA white dwarf detected, with a temperature of 88 0 00 K and log g = 7.8, lying significantly above the almost-equal-to 70 000-K observational upper limit to the DA white dwarf cooling sequenc e. The temperature and gravity are consistent with normal post-AGB evo lution. Consequently, this star occupies an important position on the white dwarf evolutionary track, having recently evolved from either th e He-rich DOs or H-rich planetary nebula central stars. We present opt ical observations of the star, and an analysis of the X-ray and EUV da ta which implies that the atmosphere of the white dwarf contains signi ficant quantities of heavy elements. This is in keeping with most othe r DA white dwarfs with temperatures greater than 50 000 K. However, as it is the only H-rich object detected above 70 000 K in the EUV, the heavy-element content is probably lower than in other known stars lyin g in this temperature regime, which were undetected at these short wav elengths.