Screening the Hipparcos-based astrometric orbits of sub-stellar objects

The combination of Hipparcos astrometric data with the spectroscopic data of putative extrasolar planets seems to indicate that a significant fraction of these low-mass companions could be brown or M dwarfs (Han et al. 2001). We show that this is due to the adopted reduction procedure, and consequently that the Hipparcos data do not reject the planetary mass hypothesis in all but one cases. Additional companions, undetected so far, might also explain the large astrometric residuals of some of these stars.