The rapid evolution of the born-again giant Sakurai’s object

The extraordinarily rapid evolution of the born-again giant Sakurai's object following discovery in 1996 has been investigated. The evolution can be traced both in a continued cooling of the stellar surface and dramatic changes in chemical composition on a timescale of a mere few months. The abun- dance alterations are the results of the mixing and nuclear reac- tions which have ensued due to the final He-shell flash which occurred during the descent along the white dwarf cooling track. The observed changes in the H and Li abundances can be ex- plained by ingestion and burning of the H-rich envelope and Li-production through the Cameron-Fowler mechanism. The rapidly increasing abundances of the light s-elements (includ- ing Sc) are consistent with current s-processing by neutrons released from the concomitantly produced 13 C. However, the possibility that the s-elements have previously been synthesized during the AGB-phase and only mixed to the surface in connec- tion with the final He-shell flash in the pre-white dwarf cannot be convincingly ruled out either. Since Sakurai's object shows substantial abundance similarities with the R CrB stars and has recently undergone R CrB-like visual fading events, the "birth" of an R CrB star may have been witnessed for the first time ever. Sakurai's object thus lends strong support for the suggestion that at least some of the R CrB stars have been formed through a final He-shell flash in a post-AGB star.