A preliminary link of the Hipparcos frame to the 3 axes of the ICRF has been
established by means of 7 stars whose positions and proper motions are known
from VLBI observations on the one hand and from the reduction of the 30-months
Hipparcos data by the FAST consortium on the other (Kovalevsky et al. 1995).
According
to Lestrade et al. (1995), the rotation angles of frame orientation at
epoch 1991.25 are in units of 1 milliarcsecond (mas)
and the rotation rates giving the relative spin are in units of 1 mas/yr
On the analogy of VLBI we undertook a pilot study using 10 stars with VLA
positions (Florkowski et al. 1985) and Hipparcos positions from 37 months of
data of the FAST consortium (Lindegren & Kovalevsky 1995). The 10 VLA link
stars were selected by applying a 2
rejection criterion to the post-fit
residuals HIP-VLA; their designations are: b Per, HD 50896, KQ Pup, FK Com, HR
5110, 
Lyr, BD +43 3571, RT Lac, AR Lac and HD 224085.
Since VLA based radio proper motions were not available, only the position link
could
be established. Using Hipparcos proper motions in place of the radio proper
motions for unavoidable epoch transformations, the following
rotation angles of the frame orientation in units of mas at epoch 1992.0 were
obtained:
Note that the quoted rotation angles and precisions result from pilot studies
on the Hipparcos link and are presented here to illustrate the capability of
VLA observations. The official link of the Hipparcos frame to the
extragalactic frame will be provided by ESA.
Due to the lower precision of the VLA positions, i.e. 30 mas as compared with roughly 1 mas for VLBI, it is not surprising that the VLA results on the rotation angles are inferior to VLBI. Nevertheless, the signs and magnitudes of the rotation angles show the same tendency for the VLBI and VLA observations. From extrapolating the VLA results a precision of 5 mas is estimated for the rotation angles when some 50 objects are involved in the least-squares adjustment process. In the course of the application of the high sensitive VLBA technique (Kellermann & Thompson 1985) an increase of number and positional accuracy of radio stars is expected and, thus, radio stars will continue to play also in future the role of intermediaries in optical and radio astrometry.