Discovery of Spectroscopic Variations in the DAB White Dwarf GD 323

We report the discovery of spectroscopic variations in GD 323, theprototypical DAB white dwarf. Simultaneous optical spectroscopic observationsover five consecutive nights of GD 323 and of PG 1234+482, a non-variablecomparison DA white dwarf of similar brightness, are used to revealquasi-periodic variations in both the hydrogen and helium absorption lines overa timescale of hours. The amplitude of the variation of the equivalent width ofHbeta is ~30 %. Moreover, the strength of the hydrogen lines is shown to varyin opposite phase from that of He I 4471. These results suggest that the modelcurrently thought to be the most viable to account for the simultaneouspresence of hydrogen and helium lines in GD 323, namely a static stratifiedatmosphere, may need to be reexamined. Instead, a model with an inhomogeneoussurface composition, resulting perhaps from the dilution of a thin hydrogenatmosphere with the underlying helium convection zone, may be a betterrepresentation of GD 323. The observed variation timescale of ~3.5 hours isconsistent with the slow rotation rate of white dwarf stars.Comment: 23 pages, 8 figures, accepted for publication in the Astrophysical Journa