Optical astrometry of Benchmark radio sources - IV. New results in the southern hemisphere

Optical positions relative to the International Reference Stars Catalogue (IRS), and therefore in the system of the FK5, have been obtained for the optical counterparts of 40 compact extragalactic radio sources south of δ ∼ +6◦. Many of these sources are being considered as possible Benchmark objects for the establishment of a quasi−inertial unified radio/optical reference frame. Precision levels as good as 0.1′′ in both coordinates were achieved. A comparison with VLBI radio positions available for these sources is presented. We give new evidence for the existence of an offset in the origin of right ascension between the radio and optical reference frames as defined at present.