DUPLICITY AMONGST HIPPARCOS STARS

Stars observed by the astrometry satellite Hipparcos may be unknown do uble stars. A subsample of those are dangerous for the extragalactic l ink of Hipparcos proper motions by long-term photographic proper motio ns, if the time base of Hipparcos is too short to detect the orbital m otion. The probability of these cases and the typical size of the phot ocentric orbital motion are estimated by Monte Carlo simulations for t he sample of 33 stars used in the Bonn extragalactic link. Both are fo und to be considerable: about 16% and 9 mas/yr respectively.