The Hipparcos optical reference frame is compared to the basic FK5 in order to determine the orientation at T 0 = 1991.25 and the global spin between the two frames. The components of the spin are significant and suggest a correction the IAU76 value of the precession constant and to a possible non-precessional motion of the equinox of the FK5. The regional errors are analysed with harmonic functions and found to be as large as 150 mas in position and 3 mas/yr in proper motion.
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Mignard et al. (1998) studied this question.