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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

Discovery of Spectroscopic Variations in the DAB White Dwarf GD 323