SIMBAD references

Precise proper motions of 69 members of the Pleiades have been determined with a median accuracy of 0.4mas/a. We demonstrate that classical reference systems like the PPM are not suited for a reduction of our data. Instead a reduction technique using positions on one reference plate is used. The comparison with two already existing catalogues shows an agreement of better than 1mas/a per star and enables for the first time a determination of the external errors of each catalogue. The external errors of the catalogues are twice as large as the internal errors but are still below the proper motion dispersion of the cluster. From all three catalogues we derive a true proper motion dispersion for the Pleiades of about 1mas/a. No indication of the existence of a velocity anisotropy was found in each catalogue. Such precise proper motions are the prerequisite to study the further dynamical evolution of the Pleiades by n-body simulations.