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

Re-processing the Hipparcos Transit Data and Intermediate Astrometric Data of spectroscopic binaries