SIMBAD references

We consider envelopes of DB white dwarfs that are not composed of pure ⁴He but rather a mixture of ³He and ⁴He. Given this assumption, the same diffusive processes that produce a relatively pure H layer overlying an He layer in the DA's should work to produce an ³He layer overlying an ⁴He layer in the DB's. We examine the relevant timescales for diffusion in these objects and compare them with the relevant evolutionary timescales in the context of the DBV white dwarfs. We then explore the consequences that ³He separation has on the pulsational spectra of DBV models. Since GD 358 is the best-studied member of this class of variables, we examine fits to its observed pulsation spectrum. We find that the inclusion of an ³He layer results in a modest improvement in a direct fit to the periods, while a fit to the period spacings is significantly improved.