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 of PG 1234 + 482 , a non-variable 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 H \beta is \sim 30 % .
Moreover , the strength of the hydrogen lines is shown to vary in opposite phase from that of He i \lambda 4471 .
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 \sim 3.5 hours is consistent with the slow rotation rate of white dwarf stars .