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Minor planets observed by Hipparcos enable us to link the Hipparcos reference frame and the dynamical one. We give here the results of this determination. However, close encounters between minor planets occured during the Hipparcos mission. Some of them are to be taken into account while determining the rotation parameters. Moreover, the modelisation of the perturbation during a close encounter is studied for one couple of minor planets, showing how it can lead to estimate the mass of a minor planet.