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While the first binary post-AGB stars were serendipitously discovered, thedistinct characteristics of their Spectral Energy Distribution (SED) allowed usto launch a more systematic search for binaries. We selected post-AGB objectswhich show a broad dust excess often starting already at H or K, pointing tothe presence of a gravitationally bound dusty disc in the system. We started avery extensive multi-wavelength study of those systems and here we report onour radial velocity and photometric monitoring results for six stars of early Ftype, which are pulsators of small amplitude. To determine the radial velocityof low signal-to-noise time-series, we constructed dedicated auto-correlationmasks. The radial velocity variations were subjected to detailed analysis todifferentiate between pulsational variability and variability due to orbitalmotion. Finally orbital minimalisation was performed to constrain the orbitalelements. All of the six objects are binaries, with orbital periods rangingfrom 120 to 1800 days. Five systems have non-circular orbits. The massfunctions range from 0.004 to 0.57 solar mass and the companions are likelyunevolved objects of (very) low initial mass. We argue that these binaries musthave been subject to severe binary interaction when the primary was a coolsupergiant. Although the origin of the circumstellar disc is not wellunderstood, the disc is generally believed to be formed during this stronginteraction phase. The eccentric orbits of these highly evolved objects remainpoorly understood. With the measured orbits and mass functions we conclude thatthe circumbinary discs seem to have a major impact on the evolution of asignificant fraction of binary systems.

Post-AGB stars with hot circumstellar dust: binarity of the low-amplitude pulsators