2000A&AS..145..161P

Only 235 entries were processed as astrometric binaries with orbits in the Hipparcos and Tycho Catalogue (Cat. I/239). However, the Intermediate Astrometric Data (IAD) and Transit Data (TD) made available by ESA make it possible to re-process the stars that turned out to be spectroscopic binaries after the completion of the Catalogue. This paper illustrates how TD and IAD may be used in conjunction with the orbital parameters of spectroscopic binaries to derive astrometric parameters. The five astrometric and four orbital parameters (not already known from the spectroscopic orbit) are derived by minimizing an objective function (χ²) with an algorithm of global optimization. This code has been applied to 81 systems for which spectroscopic orbits became available recently and that belong to various families of chemically-peculiar red giants (namely, dwarf barium stars, strong and mild barium stars, CH stars, and Tc-poor S stars). Among these 81 systems, 23 yield reliable astrometric orbits. These 23 systems make it possible to evaluate on real data the so-called ``cosmic error'' described by Wielen et al. (1997, in ESA (ed.), Hipparcos - Venice '97, ESA SP-402, p. 727), namely the fact that an unrecognized orbital motion introduces a systematic error on the proper motion. 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 ∼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.