Light curves of QSO B0909+532 in the r-band. Top panel: LT-USNO brightness records of both quasar images. The brightness of B is offset by −0.65 mag to facilitate comparison, and the new LT data correspond to our monitoring in 2012−2016. Middle panel: DLC between 2012 and 2016, and bottom panel: zoomed-in DLC around day 6300. To construct the DLC, the data of the A image have been shifted by −50 d (time delay), and then binned around the dates of the B image (using bins with a semisize of 10 d). Only bins including two or moredata have been taken into account to compute differences between A and B.