  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. To determine the radial velocity
of low signal-to-noise time-series, we constructed dedicated auto-correlation
masks. The radial velocity variations were subjected to detailed analysis to
differentiate between pulsational variability and variability due to orbital
motion. Finally orbital minimalisation was performed to constrain the orbital
elements. 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 solar mass 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. 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.
