  Previous studies have shown that many post-AGB stars with dusty disks are
associated with single-lined binary stars. To verify the binarity hypothesis on
a larger sample, we started a high-resolution spectral monitoring of about 40
field giants, whose binarity was suspected based on either a light curve, an
infrared excess, or a peculiar chemical composition. Here we report on the
discovery of the periodic RV variations in BD+46 442, a high-latitude F giant
with a disk. We interpret the variations due to the motion around a faint
companion, and deduce the following orbital parameters: Porb = 140.77 d, e =
0.083, asini=0.31 AU. We find it to be a moderately metal-poor star
([M/H]=-0.7) without a strong depletion pattern in the photospheric abundances.
Interestingly, many lines show periodic changes with the orbital phase: Halpha
switches between a double-peak emission and a PCyg-like profiles, while strong
metal lines appear split during the maximum redshift. Similar effects are
likely visible in the spectra of other post-AGB binaries, but their regularity
is not always realized due to sporadic observations. We propose that these
features result from an ongoing mass transfer from the evolved giant to the
companion. In particular, the blue-shifted absorption in Halpha, which occurs
only at superior conjunction, may result from a jet originating in the
accretion disk around the companion and seen in absorption towards the luminous
primary.
