ON THE HOT ENDS OF THE WHITE-DWARF SPECTRAL SEQUENCES - HOT WHITE-DWARFS CONTAINING HELIUM FROM THE HAMBURG-SCHMIDT SURVEY

We report on the discovery of twelve very hot white dwarf stars contai ning helium by follow-up spectroscopy of candidates selected by the Ha mburg-Schmidt survey. These stars belong to the sparsely populated spe ctral subclasses DAO, DO and PG1159 tracing the hottest ends of the wh ite dwarf spectral sequences. Eight stars show helium dominated spectr a (four DOs, two PG1159 stars and two very hot DO/sdO transition objec ts). Four stars have hydrogen dominated spectra (DAO), out of which on e is a spectroscopic binary. Metal absorption lines are detected in on e DO and one DAO star. The latter is unique and has to be classified a s the first known DAOZ star. Spectral comparison indicates that all st ars lie beyond the DB gap with T-eff exceeding 45000 K. Evidence is gi ven for T-eff as high as 100 000 K or more in some stars. With these n ew discoveries the Hamburg-Schmidt survey has increased the number of known DO and DAO white dwarfs and PG1159 stars by 55%, 30% and 40%, re spectively, an important step towards deriving statistically meaningfu l samples of the hottest white dwarfs.