The helium-atmosphere ( DB ) white dwarfs are commonly thought to be the descendants of the hotter PG 1159 stars , which initially have uniform He/C/O atmospheres .
In this evolutionary scenario , diffusion builds a pure He surface layer which gradually thickens as the star cools .
In the temperature range of the pulsating DB white dwarfs ( T _ { eff } \sim 25 , 000 K ) this transformation is still taking place , allowing asteroseismic tests of the theory .
Objective global fitting of our updated double-layered envelope models to recent observations of the pulsating DB star CBS 114 , and to existing observations of the slightly cooler star GD 358 , lead to determinations of the envelope masses and pure He surface layers that qualitatively agree with the expectations of diffusion theory .
These results provide new asteroseismic evidence supporting one of the central assumptions of spectral evolution theory , linking the DB white dwarfs to PG 1159 stars .