  The combination of HIPPARCOS measurements with suitable ground-based
astrometric data improves significantly the accuracy of the proper motions of
bright stars. The comparison of both types of data allows us also to identify
and to eliminate, at least partially, cosmic errors in the
quasi-instantaneously measured HIPPARCOS data which are caused by undetected
astrometric binaries. We describe a simple averaging method for the combination
of two independent compilation catalogues. The combination of the basic FK5
with HIPPARCOS leads to the Sixth Catalogue of Fundamental Stars (FK6). The
accuracy of the FK6 proper motions is higher than that of HIPPARCOS by a factor
of about 2 in the single-star mode, and by a factor of more than 4 in the
long-term prediction mode which takes cosmic errors into account. We present
also the error budget for a combination of the Boss General Catalogue (GC) with
HIPPARCOS data. We point out problems with known binaries, and identify an
ensemble of 'astrometrically excellent stars'.
