Multiwavelengths observations of lensed quasars: interband time delays

The time delays between brightness variations in two different optical bands have been measured for a large number of low-redshift AGN and nonlensed quasars. These time delays represent a light travel time between two different emission regions and can be used to test variability mechanisms in the source. We search for the interband delays in a sample of lensed quasars: Q2237+0305, SBS1520+530, HE2149-2745, HE1104-1805, UM673 in the redshift range 1.7 . z . 2.7. From cross-correlation analysis of light curves of the quasars we find that the brightness variations at longer wavelengths may follow the brightness variations at shorter wavelengths. The observed delays are consistent with the delays expected from reprocessing in accretion disc irradiated by the central variable source.