1 Introduction

In the process of selecting the best Benchmark objects for the establishment of a quasi-inertial unified radio/optical extragalactic reference frame (those with minimum radio-optical position residuals and minimum structure), it must be kept in mind that the measured residuals do not necessarily represent real offsets between the radio and optical emission centroids; in some cases they simply reflect the errors of the optical positions (which are at least an order of magnitude larger than those of the radio positions) and/or deviations between the radio and optical reference frames.

In spite of the above, by means of conventional ground-based observations it has been possible in the past to uncover unsuitable link objects and system deviations; subsequent removal of these sources and system realignment has allowed -in an iterative process- a continuous refinement of the selection criteria. A point has been reached in which the interpretation of more subtle differences requires that the radio and optical positions be displayed in essentially the same reference system. The Hipparcos Catalogue, which has been constructed to coincide with the ICRS, and therefore can be considered an extension to the optical domain of the radio extragalactic reference system, provides the appropriate workframe for this to be realized.

In this paper we present positions with respect to the Hipparcos Catalogue for the optical counterparts of 25 CERS selected from the list of Jauncey et al. (1989, hereafter Jauncey) and the NRL/USNO list (see e.g. Johnston et al. 1995, hereafter Johnston). The observations were made as a part of the monitoring program being carried out with the flat-field 70/100/210 cm Maksutov Astrograph at the Estación Astronómica de Cerro el Roble, in an effort to contribute to the selection of Benchmark CERS (see Costa & Loyola 1997 -hereafter Paper IV- and references therein). Precision levels as good as 0.1$\hbox{$^{\prime\prime}$}$ in both coordinates were achieved.