Context : The influence of binarity on the late stages of stellar evolution .

Aims : While the first binary post-AGB stars were serendipitously discovered , the distinct characteristics of their Spectral Energy Distribution ( SED ) allowed us to launch a more systematic search for binaries .
We selected post-AGB objects which show a broad dust excess often starting already at H or K , pointing to the presence of a gravitationally bound dusty disc in the system .
We started a very extensive multi-wavelength study of those systems and here we report on our radial velocity and photometric monitoring results for six stars of early F type , which are pulsators of small amplitude .

Methods : To determine the radial velocity of low signal-to-noise time-series , we constructed dedicated auto-correlation masks based on high signal-to-noise spectra , used in our published chemical studies .
The radial velocity variations were subjected to detailed analysis to differentiate between pulsational variability and variability due to orbital motion .
When available , the photometric monitoring data were used to complement the time series of radial velocity data and to establish the nature of the pulsation .
Finally orbital minimalisation was performed to constrain the orbital elements .

Results : All of the six objects are binaries , with orbital periods ranging from 120 to 1800 days .
Five systems have non-circular orbits .
The mass functions range from 0.004 to 0.57 M _ { \odot } and the companions are likely unevolved objects of ( very ) low initial mass .
We argue that these binaries must have been subject to severe binary interaction when the primary was a cool supergiant .
Although the origin of the circumstellar disc is not well understood , the disc is generally believed to be formed during this strong interaction phase .
The eccentric orbits of these highly evolved objects remain poorly understood .
In one object the line-of-sight is grazing the edge of the puffed-up inner rim of the disc .

Conclusions : These results corroborate our earlier statement that evolved objects in binary stars create a Keplerian dusty circumbinary disc .
With the measured orbits and mass functions we conclude that the circumbinary discs seem to have a major impact on the evolution of a significant fraction of binary systems .