Hot DB White Dwarfs from the Sloan Digital Sky Survey
  We present ugriz photometry and optical spectroscopy for 28 DB and DO white dwarfs with temperatures between 28,000K and 45,000K. About 10 of these are particularly well-observed; the remainder are candidates. These are the hottest DB stars yet found, and they populate the "DB gap" between the hotter DO stars and the familiar DB stars cooler than 30,000K. Nevertheless, after carefully matching the survey volumes, we find that the ratio of DA stars to DB/DO stars is a factor of 2.5 larger at 30,000 K than at 20,000 K, suggesting that the "DB gap" is indeed deficient and that some kind of atmospheric transformation takes place in roughly 10% of DA stars as they cool from 30,000 K to 20,000 K. 
Daniel J. Eisenstein, James Liebert, Detlev Koester, S.J. Kleinmann,   Atsuko Nitta, Paul S. Smith, J.C. Barentine, Howard J...