1994A&A...289..673T

We have studied flux variations in the optical and at radio wavelengths of a sample of extragalactic sources. Even though similar analyses have been done earlier, this is the first extensive study of connections between long-term optical and high radio frequency (from 22 GHz up to 230 GHz) variability in a large sample (22 sources) of active galactic nuclei (AGN). For the correlation analysis we used the discrete correlation function complemented by visual comparison and concentrated on finding correlations with time delays of about 1000 days and less, since any time delays longer than some hundred days are very hard to verify. In 10 out of the 22 sources we found a clear radio-optical correlation, and in as many as 6 sources the optical and high frequency radio variability was simultaneous. We suggest that the main reason for not finding significant correlations in the rest of the sample sources is the lack of sufficient radio and especially optical data.