Spectroscopic analyses of hot pre-white dwarfs, i.e. subluminous O and B stars, are presented. In the B-type stars, the resulting abundance patterns are indicative of atmospheric diffusion. Amongst the O-type subdwarfs, a new group of comparatively luminous stars is identified. Their position in the HR-diagram suggests that, unlike the "classical" sdOs, they are in a post-AGB stage of evolution. Spectroscopic evidence is presented showing that the born-again post-AGB star scenario of Iben et al. (1983) can explain their origin.
The Impact of Very High S/N Spectroscopy on Stellar Physics
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