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Connection of the HIPPARCOS Catalogue to the Extragalactic Reference Frame

The IAU decided to replace the FK5 reference system by a celestial reference system based upon the positions of extragalactic radio-sources. This system now exists and is materialised by the International Celestial Reference Frame, ICRF. It was in addition requested that the Hipparcos catalogue be given in this system. The coordinate axes in which positions and proper motions are given (as determined by the Hipparcos data reduction procedures) constitute a provisional frame which is to some extent arbitrary. It was therefore necessary to determine the rotation and the yearly spin that must be applied to put the catalogue in the ICRF. Eleven teams, participated in this work using several diierent techniques: radio-interferometry, photographic plates, star catalogues linked to galaxies, HST observations, analysis of the Earth rotation determined with Hipparcos stars. The synthesis of their results by two diierent methods gave very similar results. The alignment of the Hip-parcos catalogue with the ICRS is realised in position within 0.6 mas at the central epoch 1991.25 and in spin, within 0.25 mas per year.