Flux and color variations of the multiply imaged quasars HE0435 and UM673

We present photometric studies of the gravitationally lensed QSOs UM673 and HE0435. Concerning the doubly imaged quasar UM673, we are presently carrying out a preliminary analysis of the observations. Regarding the quadruply imaged quasar HE0435, the analysis is complete. The variations of this lensed quadruplet show that the source is intrinsically variable. Furthermore, one component is also very likely affected by microlensing. The significant color variations observed for the four lensed images are also consistent with variations produced by the source, and not by the lens. Our flux and color analysis are based upon different methods (image subtraction and PSF fitting) for studying the multi-band variability.