GRAVITATIONAL LENSING OF ACTIVE GALACTIC NUCLEI

Most of the known cases of strong gravitational lensing involve multip le imaging of an active galactic nucleus. The properties of lensed act ive galactic nuclei make them promising systems for astrophysical appl ications of gravitational lensing; in particular, they show structure on scales of milliseconds of are to tens of seconds of are, they are v ariable, and they are polarized. More than 20 cases of strong gravitat ional lenses are now known, and about half of them are radio sources. High-resolution radio imaging is making possible the development of we ll-constrained lens models. Variability studies at radio and optical w avelengths are beginning to yield results of astrophysical interest, s uch as an independent measure of the distance scale and limits on sour ce sizes.