Why are the BL Lac objects AO 0235 + 164 and PKS 0537 - 441 not multiply imaged by gravitational lensing?

Recently, galaxies have been found superposed on a few high-redshift BL Lac objects. These galaxies are presumed to be foreground objects, not directly associated with the sources. Using the fact that the BL Lac objects are not multiply imaged as a result of gravitational lensing, we obtain interesting constraints on the parameters of the respective foreground galaxies. In particular, from the observed angular separation of the galaxy and the BL Lac object, and an assumed redshift and velocity dispersion of the galaxy, we derive lower limits for the core radius of the galaxy. We analyse a circularly symmetric galaxy model as well as the case when there is an external symmetry-breaking shear, which is expected if the galaxy lies within a cluster. (author)