2001A&A...367..719C

Optical positions relative to the Hipparcos Catalogue (ESA 1997, Cat. I/239 have been obtained for the optical counterparts of 28 faint (B∼20-23) southern compact extragalactic radio sources (CERS). Most of these sources are not adequately observable (or simply not visible) by means of direct photography with a conventional wide-field telescope, so the positions were determined using a multi-step procedure involving CCD and photographic observations. This method responds both to the need of attaining small positional errors and the need to refer the positions to a standard astrometric system. Estimated precisions as good as 60 mas were achieved. For 13 of the above objects, namely 0214-522, 0614-349, 0700-465, 0823-500, 1120-274, 1143-331, 1347-218, 1349+027, 1817-254, 1829-718, 1920-211, 2139+028 and 2211-388, we are proposing a new optical identification. A comparison with VLBI radio positions available for these sources is presented. The residuals obtained are in most cases consistent with the precision of our optical data, and provide an evaluation of these faint objects as possible radio/optical frame link sources (or their identification as astrophysically interesting astrometric outliers). Fifteen of the CERS observed are either defining or candidate fiducial objects of the present realization of the International Celestial Reference System (ICRS, Ma et al., 1998AJ....116..516M), so the optical data we have obtained for them is potentially useful to help mantain and possibly improve the current link of the Hipparcos reference frame to the ICRS.