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Photometric optical data of QSO 0957+561 covering the period 1984-99 areanalyzed to discern between the two values of the time delay (417 and 424 days)mostly accepted in the recent literature. The observations, performed by groupsfrom three different institutions-Princeton University, Harvard-SmithsonianCenter for Astrophysics, and Instituto de Astrofisica de Canarias-and includingnew unpublished 1998-9 data from the IAC80 Telescope, were obtained in fivefilters (V, R, I, g, and r). The different light curves have been divided intoobservational seasons and two restriction have been applied to better calculatethe time delay: (i) points with a strange photometric behavior have beenremoved; and (ii) data sets without large gaps have been selected. Simulateddata were generated to test several numerical methods intended to compute thetime delay. The methods giving the best results-the discrete correlationfunction, delta-square, z-transformed discrete correlation function, and linearinterpolation-were then applied to real data. A first analysis of the 23different time delays derived from each technique shows that the time delaymust be into the interval 420-424 days. From our statistical study, a mostprobable value of 422.6 +/- 0.6 days is inferred.Comment: 27 pages including 10 tables and 5 color figures. Accepted for  publication in Ap

Time Delay in QSO 0957+561 From 1984–1999 Optical Data