The Sixth Catalogue of Fundamental Stars (FK6) and the Problem of Double   Stars
  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. 
R. Wielen, H. Schwan, C. Dettbarn, H. Jahreiss, H. Lenhardt