2005ApJ...623.1076P

We report the discovery of spectroscopic variations in GD 323, the prototypical DAB white dwarf. Simultaneous optical spectroscopic observations over five consecutive nights of GD 323 and PG 1234+482, a nonvariable comparison DA white dwarf of similar brightness, are used to reveal quasi-periodic variations in both the hydrogen and helium absorption lines over a timescale of hours. The amplitude of the variation of the equivalent width of Hβ is ∼30%. Moreover, the strength of the hydrogen lines is shown to vary in opposite phase from that of He I λ4471. These results suggest that the model currently thought to be the most viable to account for the simultaneous presence of hydrogen and helium lines in GD 323, namely, a static stratified atmosphere, may need to be reexamined. Instead, a model with an inhomogeneous surface composition, resulting perhaps from the dilution of a thin hydrogen atmosphere with the underlying helium convection zone, may be a better representation of GD 323. The observed variation timescale of ∼3.5 hr is consistent with the slow rotation rate of white dwarf stars.