  Photometric optical data of QSO 0957+561 covering the period 1984-99 are
analyzed to discern between the two values of the time delay (417 and 424 days)
mostly accepted in the recent literature. The observations, performed by groups
from three different institutions-Princeton University, Harvard-Smithsonian
Center for Astrophysics, and Instituto de Astrofisica de Canarias-and including
new unpublished 1998-9 data from the IAC80 Telescope, were obtained in five
filters (V, R, I, g, and r). The different light curves have been divided into
observational seasons and two restriction have been applied to better calculate
the time delay: (i) points with a strange photometric behavior have been
removed; and (ii) data sets without large gaps have been selected. Simulated
data were generated to test several numerical methods intended to compute the
time delay. The methods giving the best results-the discrete correlation
function, delta-square, z-transformed discrete correlation function, and linear
interpolation-were then applied to real data. A first analysis of the 23
different time delays derived from each technique shows that the time delay
must be into the interval 420-424 days. From our statistical study, a most
probable value of 422.6 +/- 0.6 days is inferred.
