Annu. Rev. Astron. Astrophys. 1981. 19: 373-410 Copyright © 1981 by Annual Reviews. All rights reserved

4.4 Optical and Infrared Variability

In general. there is no detailed correlation between flux density outbursts observed at radio and at IR-optical wavelengths (Pomphrey et al. 1976), although sources that show large radio variations are usually included in the class of so-called Optically Violent Variables. In a few outstanding cases, however, such as the BL Lac objects OJ 287 (Pomphrey et al. 1976) and 0235 + 16 (Balonek & Dent 1980) and in the quasar 1921-29 (Gilmore 1980), there is a clear relation between the activity at radio and optical wavelengths, implying a relation between the emission mechanisms at the two wavelengths and near equivalence of the emitting volumes.