1 Introduction

The determination of the position of radio emitting stars in the Hipparcos reference frame (ESA 1997), as represented by the Tycho (ESA 1997) catalogue and the ACT (Urban et al. 1998a) system of proper motions, enables, in principle, to study the relative orientation between that frame and the International Celestial Reference Frame - ICRF (Kovalevski et al. 1997). Radio stars represent an important and unique class of objects that is directly observable by the primary instruments that define the radio and optical frames (Lindegren & Kovalevsky 1995; Stone 1997). The advantage of using bright stars, whose radio positions are directly tied to nearby ICRF sources, however, is hampered by the errors of the stars' proper motions (Andrei et al. 1995). Therefore the interest on the high-precision observations of radio emitting stars belonging to the Hipparcos reference frame is justified. They must be made on a continuing basis leading to refinement of the proper motions.

Densification of the Hipparcos reference frame, on the other hand, is in itself desired, since many observing programs require fainter and more numerous reference stars than those in the Tycho catalogue. The Twin Astrographic Catalog - TAC (Zacharias et al. 1996) represents an important step in this direction. In the present paper, its initial version (TAC 1.0) is used. A further version shall present an improved set of proper motions and is going to be oriented in the Hipparcos system. We point however that the epochs closeness between the considered catalogues and our observations decreases the errors due to faulty proper motions. Thus we can extract from our data, a sample of the internal accuracy for the TAC in the observed zone.

At the IAG/USP CCD Meridian Circle, in Valinhos, São Paulo ($\varphi$ = -23$^\circ$ 00' 06'', $\lambda$ = +3$^{\rm h}$ 07$^{\rm m}$52.2$^{\rm s}$, h = 850 m), a program for the observation of 640 confirmed or suspected radio stars, taken from the list of Wendker (1995), is being currently run. The selected stars are contained in the declination zone $-77^\circ$ to +30$^\circ$ and in the magnitude range from 9 to 14. The instrument has an aperture of 0.19 m and focal length of 2.59 m. It is equipped with a Thomson 7895A CCD camera, which has a 512 $\times$ 512 pixels matrix. The pixels are squares of 19 $\mu$m side, that correspond to 1.5 arcsec on the sky. The observations are proceeded in drift scanning mode, that is, leaving the field parading across the matrix and integrating the resulting signal. The equivalent exposure time is 51$^{\rm s}$ sec$\delta$. All the observations have been made on fields of 13 arcmin in declination by 30 minutes in right ascension, enabling an average of 36 Tycho stars and a few Hipparcos stars in each field. The detection of objects is made at the level of 3 sigma above the sky background. The center of the objects are then found by a two-dimensional Gaussian fit. The reduction of the (x, y) so obtained is made through a 3 constants model for each direction independently. A comprehensive account of the instrument capabilities and mode of operation is given by Viateau et al. (1999).

We present the results from the observation of 16 stars, all within the TAC range of declinations. Of these stars, 8 belong to the TAC catalogue, 11 belong to the Tycho catalogue and 8 to the Hipparcos catalogue. Thus, 12 stars appear in at least one of the catalogues and 5 in all three. In the observed fields 573 Tycho stars and 566 TAC stars are found and also have their positions re-measured relatively to all three frames.

Finally, positions from the new reduction of the Astrographic Catalogue (Urban et al. 1998b) were searched for the radio stars, leading to a new determination of their proper motions.