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With Gaia, it will become possible to directly link the radio and opticalreference frames using a large number of common objects. For the most accurateradio-optical link, it is important to know the level of spatial coincidencebetween the quasars' optical positions, and the radio positions determined byVery Long Baseline Interferometry (VLBI) observations. The "outlier" objects,for which the positions are significantly offset at the two differentelectromagnetic wavebands, may be of astrophysical interest as well. Here wepresent a case study to compare the radio positions of ~800 quasars common inthe second realization of the International Celestial Reference Frame (ICRF2)and in the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) catalogue.Compared to the radio ICRF2, the SDSS provides two orders of magnitude lessaccurate astrometric data in the optical. However, its extensive sky coverageand faint magnitude limit allow us to directly relate the positions of a largesample of radio sources. This way we provide an independent check of theoverall accuracy of the SDSS positions and confirm that the astrometriccalibration of the latest Data Release 8 (DR8) is poorer than that of the DR7.We find over 20 sources for which the optical and radio brightness peaks areapparently not coincident at least at the 3-sigma level of SDSS DR7 positionalaccuracy, and briefly discuss the possible causes, including dual activegalactic nuclei.

Radio-optical outlier quasars - a case study with ICRF2 and SDSS