  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 ~ 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.
