1997A&A...324..991B

Based on a set of evolutionary calculations for thermally pulsating post-AGB models we introduce a robust method to measure FG Sge's mass which was found to be 0.61M_☉. The corresponding evolutionary timescale is consistent with the expansion age of the planetary nebula. Assuming that FG Sge's surface was already enriched with heavy elements on the AGB we propose that during the central-star evolution these elements were removed from the superficial radiative layers due to, e.g., dust/gas separation, leaving the deeper convective layers unchanged. During the flash the outward moving envelope convection mixed the stored heavy elements back to the surface.