The equatorial cartesian axes of the Hipparcos Catalogue have been fixed relative to those of the International Celestial Reference Frame (ICRF) with an accuracy of plus or minus 0.6 milliarcseconds (mas) at the epoch 1991.25 and with a constraint of plus or minus 0.25 mas/yr on their rotation. The maintenance of the Hipparcos frame will depend on periodic checks of its alignment to the ICRF. One of the most effective ways of doing this will be to compare the radio positions of Hipparcos stars with the Hipparcos optical positions brought forward with Hipparcos proper motions. From the radio positions of 11 Hipparcos radio stars, the MERLIN connected-element radio interferometer confirmed that the alignment of the Hipparcos frame at epoch ~1994 was within 2.3 mas of the ICRF. Ten years from the mean epoch of Hipparcos (2001) the misalignment of the Hipparcos frame to the ICRF could reach 3 mas. By doubling the number of measurable radio stars to ~20 and improving corrections for atmospheric phase errors, MERLIN will be capable of testing the alignment of Hipparcos to the ICRF with an accuracy of about plus or minus 1 mas. We discuss some new MERLIN positions of radio stars, which is the next step towards the realization of this objective.
IAU Joint Discussion
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