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 .
We have obtained dual-site observations of the pulsating DB star CBS 114 , to complement existing observations of the slightly cooler star GD 358 .
We recover the 7 independent pulsation modes that were previously known , and we discover 4 new ones to provide additional constraints on the models .
We perform objective global fitting of our updated double-layered envelope models to both sets of observations , leading 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 .