6 Conclusions

The availability of the Hipparcos Intermediate Astrometric Data and, when they exist, the Transit Data, allow to improve upon the existing Hipparcos catalogue. Once a new or revised spectroscopic binary orbit becomes available, it can be used to update the Hipparcos astrometric parameters and, sometime, to derive the corresponding astrometric orbit. In the latter case, the orbital inclination is obtained, thus allowing to improve our knowledge of the binary system.

The optimization method used in the present paper yields a solution in all cases, and it is therefore important to perform consistency checks. One test which may be applied in all cases involves the comparison of the arguments of the periastron and of the semi-amplitude of the radial-velocity curve, as derived from either the spectroscopic orbit or the astrometric one. The comparison of the semi-amplitude of the radial-velocity curve involves however the inclination of the orbit which is not always very well determined.

In this context, chemically-peculiar red stars (CPRS) like barium, Tc-poor S and CH stars are interesting because they provide their own consistency checks. Indeed, beside the orbit which can be determined as for any other spectroscopic binary, the mass of both components may be guessed with some confidence. These mass estimates based on astrophysical considerations may then be compared to the mass of the system derived from the astrometric orbit using Kepler's third law.

From a sample of 81 CPRS spectroscopic systems whose orbits became available after completion of the Hipparcos catalogue, we have derived 23 reliable astrometric orbits. This shows that the number of Hipparcos entries for which an orbital solution may be obtained is much larger than suggested by the number of existing entries in the DMSA/O.

Updated astrometric parameters from this particular sample have shown that the Hipparcos-catalogue parallaxes are not reliable for systems with orbital periods close to 1 yr. Similarly, the Hipparcos proper motions are not reliable for unrecognized binaries with periods in the range 3 to about 8 yr, as already suspected by Wielen et al. (1999).

The orbit-determination methods based on the IAD and TD being completely different, systems for which the two sets of data are available can be used to assess the consistency of the solutions derived from both sets with different methods. It turns out that the two sets of results are generally in good agreement.

Acknowledgements

We thank P. North for communicating us the orbital elements of dwarf Ba stars in advance of publication. We also thank two anonymous referees for their very constructive comments.