New Spectral Analyses of Pre-white Dwarfs

The very hot, H-deecient PG 1159 stars are found in the post-AGB region of the Hertzsprung-Russell diagram. Most likely the \born-again" post-AGB scenario is valid for their evolution. It predicts a nal He-ash which brings back the star to the AGB, followed by a second (He burning) descent from the AGB where a superwind may take oo the entire H-rich and most of the He-rich envelope and lay bare intershell layers, where Ne is the fourth abundant element after He, C, and O. In order to support this hypothesis, we present a search for the Ne VII 3644 A absorption line in several of these stars. The two presently known O(He) central stars of planetary nebulae, K 1-27 and LoTr 4, exhibit almost pure He II absorption line spectra. They are found in the post-AGB region of the Hertzsprung-Russell diagram just amongst the PG 1159 stars which in contrast show additional strong C IV lines. We present an analysis of three recently discovered O(He) stars, HS 0742+6520, HS 1522+6615 and HS 2209+8229, for which no associated planetary nebulae have been detected. It is likely that there exists an evolutionary channel for H-deecient post-AGB stars, \O(He) stars ! DO white dwarfs", which is separate from to the well established \Wolf-Rayet WC] central stars ! PG 1159 stars ! DO white dwarfs" evolutionary sequence.