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

One of the most important results of the ROSAT all-sky X-ray and EUV surveys has been the discovery of a large number of previously uncatalogued white dwarfs. One of these, RE 1738 + 665, is found to be the hottest hydrogen-rich DA white dwarf detected, with a temperature of 88 000 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 sequence. The temperature and gravity are consistent with normal post-AGB evolution. Consequently, this star occupies an important position on the white dwarf evolutionary track, having recently evolved from either the He-rich DOs or H-rich planetary nebula central stars. We present optical observations of the star, and an analysis of the X-ray and EUV data which implies that the atmosphere of the white dwarf contains significant quantities of heavy elements. This is in keeping with most other 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 lying in this temperature regime, which were undetected at these short wavelengths.