  The Sixth Catalogue of Fundamental Stars (FK6) combines the ground-based
astrometric data of the basic fundamental stars, obtained over more than two
centuries and summarized in the FK5, with the observations of the HIPPARCOS
astrometry satellite. This combination provides the most accurate proper
motions available at present. There are two main reasons why the ground-based
observations are able to improve the HIPPARCOS proper motions considerably: (1)
The lower positional accuracy of the ground-based observations is compensated
by a much longer period of observations, covering more than 200 years for most
FK6 stars; (2) Due to undetected astrometric binaries, the HIPPARCOS proper
motions, measured 'instantaneously' during a period of about 3 years only, can
deviate significantly from the long-term proper motions, while the ground-based
results of the FK5 provide already fairly 'time- averaged' data. Hence the
ground-based results allow to identify and to correct, at least partially,
'cosmic errors' in the HIPPARCOS proper motions. The proper motions of the FK6,
and hence the positions predicted from the FK6 data, are significantly more
accurate than the data given in the HIPPARCOS Catalogue and in the FK5. For
truly single stars, the accuracy is improved by a factor of about 2 by the FK6.
For the ensemble of apparently single stars, in which the HIPPARCOS proper
motions suffer from cosmic errors due to undetected astrometric binaries, the
gain is even higher, by a factor of more than 4. Double stars pose special
problems which are handled rather individually in the FK6. Among all the FK6
stars, a subset of 'astrometrically excellent stars' is identified for
high-precision astrometry, avoiding as far as possible a disturbing binary
nature of the objects.
