Radio stars are suitable objects for an immediate connection of stellar reference frames to radio reference frames and vice versa. From the positions of the stars in both frames follows the orientation of one frame with respect to the other at some specified epoch; the spin is zeroized by means of the proper motions. At present, in view of the limited number of radio stars the determination of the orientation and spin parameters is best performed by rigid rotations. This mode of execution is even more suggested when it can be taken for granted that the stars representing the frames constitute rigid spheres.
A practical application of this method is the determination of the link of the provisional Hipparcos frame to the extragalactic reference frame based on radio sources (Lindegren & Kovalevsky 1995). This frame has been established by IERS (IERS 1994) and is going to be adopted by IAU as the conventional celestial reference frame (ICRF).
The IAU Joint Working Group on Reference Frames (IAU 1992, 1996) at its meeting in Hamburg, 1994 March 28-29, has tentatively selected some 50 radio stars as candidates for setting up the link to the ICRF and for maintaining the link. Clearly, this explains the important role of radio stars not only for the presence but also in future. It is therefore not surprising that a growing body of opinion among the astronomical community called for a revision of the existing data on radio stars.
Since radio stars are found among a large variety of classes of stars such as single stars, binaries, variable stars, masers etc., the stellar data are widely scattered, a fact which suggests a consistent data assembling and the compilation of a catalogue of radio stars. In contrast to the optical data, an extensive collection of radio properties has been made by Wendker (1995). The search of radio positions of high precision depends almost exclusively on the VLBI, VLA and MERLIN networks.
Our catalogue contains updated astrometric and astrophysical data of the
radio stars chosen by the working group in 1994 augmented by a dozen of stars
that have become eligible for the link in the meantime. Further, we give keys
that allow the retrieval of detailed radio information and the identification
of the interferometry networks, which have the individual star on their
observing programme. All in all the catalogue comprises 66 objects. Because of
their status they may be called "the cardinal radio stars for linking celestial
frames''. Their sky distribution is plotted in Fig. 1 (click here). The region below 
of declination is missing among the stars recommended by the Working Group
since the radio stars confirmed in this part of the sky are not yet
sufficiently explored by radio interferometry.
At least for some time the observing efforts will concentrate on the cardinal radio stars aiming at the determination of precise radio proper motions for the entire set. To extend the set of cardinal radio stars one may take advantage of a fund of confirmed and suspected radio stars for astrometry which are listed in a former catalogue by Walter et al. (1990) including 186 objects most of them being Hipparcos stars. The majority of the cardinal radio stars are among these objects, but many stars of the former catalogue are either suspected radio stars due to a low flux density or their radio positions are derived from single dish observations thus leaving unresolved radio/optical identification problems.
Figure 1: Cardinal radio stars for the link of optical reference frames to the
extragalactic reference frame. The sky distribution is shown in equatorial
coordinates, the dotted line marks the galactic equator. Circles and crosses
refer to stars with optical positions and, apart from a few exceptions, with
positions from radio interferometry. More specifically, crosses indicate stars
with VLBI positions
In Sect. 2 the potential of radio stars for connecting frames is briefly exemplified by the results obtained from preliminary Hipparcos catalogue versions. Section 3 describes the organization of the catalogue which finally follows in the annex.