Properties of EUV/Soft-X-Ray-Selected White Dwarfs

I review the atmospheric and stellar properties of a sample of ~ 150 hydrogen-rich white dwarfs drawn from the ROSAT XRT and Extreme Ultraviolet Explorer catalogs. The new sample helps redefine the mass distribution of white dwarf stars, and, in particular, the existence of over a dozen objects in our sample with masses in excess of 1.1 M_sun challenges stellar evolution theory which predict a mass distribution narrowly clustered around 0.6 M_sun. The presence of a magnetic field in four out of fourteen ultramassive white dwarfs also distinguishes these objects from the general population of white dwarfs for which only four objects in a hundred has a detectable field. I also discuss the temperature and age distribution of the sample and present a comparison with optical surveys.