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In this contribution we report on our radial-velocity monitoring of optically bright, high-latitude supergiants that appear to be in a post-AGB evolutionary stage. Binarity is a widespread phenomenon among our sample stars. More precisely: all objects with a near-IR excess in their energy distribution turn out to be binaries while the fraction of binaries in our program stars with only a far-IR excess is very small. The orbital periods, the often non-zero eccentricities, and the sometimes large mass functions set strong constraints on the previous evolution in which mass transfer must have been an important ingredient. We have accumulated observational evidence that the presence of a circum-binary dusty disk has an important dynamical and sometimes even chemical influence on the binary and its evolution. Some objects with a high mass function still defy an explanation.
Systematic radial velocity monitoring programs revealed that all optically bright, high-latitude post-AGB stars that show a near-IR excess in their energy distribution are binaries (see the contribution of R. Waters et al. in these proceedings). A standard AGB evolution for the shorter period objects seems, however, impossible since the orbits are too small to accommodate an AGB star! Non-standard phenomena connected to the specific binary nature have to be invoked in order to understand these systems.
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