We have presented absolute proper motions of 48 Hipparcos from measurements of photographic plates. These data have been included in the calibration of the Hipparcos proper motion system (Kovalevsky et al. 1997). The accuracy of one single proper motion is of the order of 2 to 3 mas/a.
Acknowledgements
We are indebted to many staff members of the observatories of Bordeaux, Bruxelles, Heidelberg, Leiden, RGO and Vienna for continuous support, which enabled us to use their plates. We thank A. Karafistan, C. Ducourant for the measurements of some of the plates and D. Sinachopoulos for providing us the program used in Sect. 4.