Evidence of thin helium envelopes in PG1159 stars

We present evidence that PG1159 stars could harbour He--rich envelopes substantially thinner than those predicted by current evolutionary models with current estimates of mass loss, which may be attributable to an extensive mass--loss episode during the born--again AGB phase. Specifically, we show that the models with thin He--rich envelopes predict remarkably large magnitudes of the rates of period change of the trapped and untrapped modes observed in the pulsating star PG 1159$-$035. This is a consequence of the much shorter evolutionary timescale of the models with thin He--rich envelopes during the low--gravity PG1159 regime. Our findings are particularly interesting in view of the suggestion of an evolutionary link between the helium--deficient PG1159 star H1504+65 and the recently discovered white dwarfs with almost pure carbon atmospheres.