2002A&A...388..320W

Asteroseismological analysis of the Whole Earth Telescope observations of the prototype variable DB white dwarf GD358 indicates a possible composition interface at a depth of about 10^–6 M_☉, which has usually been assumed to be the He/C transition zone. Such a small He layer is in contradiction with canonical evolution theory. Recently Montgomery & Winget (2000BaltA...9...23M) proposed that the transition zone is in fact the transition between the isotopes ³He and ⁴He, which are separated by the same sedimentation process separating hydrogen and helium in the DA white dwarfs. Therefore, the visible surface helium should be ³He. We have tested this hypothesis spectroscopically on another variable DB using the known relative isotopic shifts of up to 0.5Å. The spectra are consistent with a pure ⁴He atmosphere, making a ³He/⁴He interface a very unlikely explanation for the mode spectrum of GD 358.