SDSS White Dwarfs with Spectra Showing Atomic Oxygen and/or Carbon Lines

We discuss 18 white dwarfs, one of which (G227-5) was previously known,whose SDSS spectra show lines of neutral and/or singly ionized carbon.At least two and perhaps four show lines of neutral or singly ionizedoxygen. Apart from the extremely hot ``PG 1159'' stars, these are thefirst white dwarfs with photospheric oxygen detected in their opticalspectra. The photometry strongly suggests that these stars lie in the11,000-30,000 K temperature range of the helium-atmosphere DB whitedwarfs, though only one of them shows weak neutral helium lines in thespectrum. Trigonometric parallaxes are known for G227-5 and another,previously known white dwarf (G35-26) showing atomic carbon lines, andthey indicate that both are massive stars. Theoretical arguments suggestthat all members of this class of rare white dwarfs are massive (~1M_solar), and this finding could explain the paucity ofmassive DB white dwarfs.