1996A&A...311L..17H

We report on the discovery of twelve very hot white dwarf stars containing helium by follow-up spectroscopy of candidates selected by the Hamburg-Schmidt survey. These stars belong to the sparsely populated spectral subclasses DAO, DO and PG1159 tracing the hottest ends of the white dwarf spectral sequences. Eight stars show helium dominated spectra (four DOs, two PG1159 stars and two very hot DO/sdO transition objects). Four stars have hydrogen dominated spectra (DAO), out of which one is a spectroscopic binary. Metal absorption lines are detected in one DO and one DAO star. The latter is unique and has to be classified as the first known DAOZ star. Spectral comparison indicates that all stars lie beyond the DB gap with T_eff exceeding 45000K. Evidence is given for T_eff as high as 100000K or more in some stars. With these new discoveries the Hamburg-Schmidt survey has increased the number of known DO and DAO white dwarfs and PG1159 stars by 55%, 30% and 40%, respectively, an important step towards deriving statistically meaningful samples of the hottest white dwarfs.