The effect of 3-He diffusion on the pulsational spectra of DBV models
  We consider envelopes of DB white dwarfs which are not composed of pure 4-He, but rather a mixture of 3-He and 4-He. Given this assumption, the same diffusive processes which produce a relatively pure H layer overlying a He layer in the DA's should work to produce a 3-He layer overlying a 4-He layer in the DB's. We examine the relevant timescales for diffusion in these objects, and compare them to the relevant evolutionary timescales in the context of the DBV white dwarfs. We then explore the consequences which 3-He 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 a 3-He layer results in a modest improvement in a direct fit to the periods, while a fit to the period spacings is significantly improved. 
M. H. Montgomery, T. S. Metcalfe, and D. E. Winget