A possible model of variable radio sources with "superluminal" motions of VLBI components.

The basic properties of 'superluminal' VLBI components in compact variable extragalactic radio sources are presented. A version of the 'screen' model is considered whereby 'superluminal' components move with the phase velocities of the region of interaction between precessed relativistic plasma jets and heterogeneities of the circumnuclear medium. Within the framework of the present model, the effect of 'superluminal' motions should always be accompanied, in a wide range of wave-lengths, by emission variability caused by the entrance of separate gas structures into precession 'beams'. It is concluded that 'superluminal' motions can be observed in ranges other than the radio range and that emission variability appears in the circumnuclear region.