Microlensing variability in time-delay quasars

We have searched for microlensing variability in the light curves of five gravitationally lensed quasars with well-determined time delays: SBS 1520+530, FBQ 0951+2635, RX J0911+0551, B1600+434 and HE 2149-2745. By comparing the light curve of the leading image with a suitably time offset light curve of a trailing image we find that two (SBS 1520+530 and FBQ 0951+2635) out of the five quasars have significant long-term (~years) and short-term (~100 days) brightness variations that may be attributed to microlensing. The short-term variations may be due to nanolenses 10^-4-10^-3~M_⊙, relativistic hot or cold spots in the quasar accretion disks, or coherent microlensing at large optical depth.