  The Intermediate Astrometric Data (IAD) and Transit Data (TD) made available
by ESA after the Hipparcos mission make it possible to re-process the
observations of any Hipparcos entry. This paper illustrates how TD and IAD may
be used in conjunction with the orbital parameters of spectroscopic binaries to
improve the astrometric parameters of binaries considered as single stars in
the original reduction process. That approach has been applied to 81 dwarf
barium stars, strong and mild barium stars, CH stars, and Tc-poor S stars for
which spectroscopic orbits became available recently. Among these 81 systems,
23 yield reliable astrometric orbits, thus making it possible to evaluate on
real data the impact of an unrecognized orbital motion on the proper motion
(Wielen, 1997, A&A 325, 367). Comparison of the proper motion from the
Hipparcos catalogue with that re-derived in the present work indicates that the
former are indeed far off the present value for binaries with periods in the
range 3 to about 8 years. Hipparcos parallaxes of unrecognized spectroscopic
binaries turn out to be reliable, except for systems with periods close to 1
year, as expected. Finally, we show that, even when a complete orbital
revolution was observed by Hipparcos, the inclination is unfortunately seldom
precise.
