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▪ Abstract Advances in wide-angle astrometric measurements of three to four orders of magnitude in the last thirty years have resulted in a redefinition of the fundamental astronomical reference frame. This new frame, the International Celestial Reference Frame (ICRF), is based on the radio positions of 212 compact extragalactic radio sources. The ICRF defines the direction of the axes of the International Celestial Reference System (ICRS) with a precision of approximately 20 μas. At optical wavelengths, the Hipparcos catalog is the realization of this frame. The precision with which the ICRF is now determined requires that the ICRS models for precession, nutation, and others, be revised. Increases in the precision of measurements from astrometric space missions will further improve the celestial reference frame and may require its redefinition within the next ten years. These improvements will again challenge the models for the celestial reference system.

Radio stars play a key role in establishing the link between optical reference frames and the conventional celestial reference frame based on extragalactic radio sources. The relevant astrometric, astrophysical and radio quantities are compiled of 66 cardinal radio stars currently suited to frame connection and maintenance of the link. The catalogue entries are supplied with ample bibliographical codes and annotations for easy data retrieval.

The construction of a new extragalactic all-sky reference frame based on groundbased high precision optical and radio astrometry of selected compact radio sources and radio stars has been started in 1987 as a joint longterm project involving several astrometric facilities in both hemispheres. At present this system is based on a masterlist of about 400 extragalactic sources, mostly displaying optical counterparts and about 100 radio stars. A progress report is given with special view to an application for the HIPPARCOS extragalactic reference link.

A catalog of 233 strong compact extragalactic radio sources which display optical counterparts has been prepared by the working group as a basis for the establishment of an almost inertial extragalactic reference frame. Optical properties of the catalog sources are summarized and further requirements for a refinement and extension of the present data are discussed.

The Very Large Array (VLA) has made possible the measurement of the precise positions of the radio emission associated with stars. This allows the direct comparison of the optical reference frame (FK4) with the radio reference frame which is defined by the quasi-absolute positions of extragalactic radio sources. This comparison is limited by the small number of bright stars that display detectable radio emission and the lack of knowledge of the precise coincidence of the radio emission with the optical photocenter of the star. Since the VLA is the most sensitive astrometrically capable radio telescope, positions of the largest number of stars north of declination -20 degrees can be measured. The accuracy of the positions on the extragalactic reference frame should approach a milliarcsecond.

A catalog of 233 strong compact extragalactic radio sources which display optical counterparts has been prepared by the working group as a basis for the establishment of an almost inertial extragalactic reference frame. Optical properties of the catalog sources are summarized and further requirements for a refinement and extension of the present data are discussed.