  We extend the gr-band time coverage of the gravitationally lensed double
quasar Q0957+561. New gr light curves permit us to detect significant intrinsic
fluctuations, to determine new time delays, and thus to gain perspective on the
mechanism of intrinsic variability in Q0957+561. We use new optical frames of
Q0957+561 in the g and r passbands from January 2005 to July 2007. These frames
are part of an ongoing long-term monitoring with the Liverpool robotic
telescope. We also introduce two photometric pipelines that are applied to the
new gr frames of Q0957+561. The transformation pipeline incorporates
zero-point, colour, and inhomogeneity corrections to the instrumental
magnitudes, so final photometry to the 1-2% level is achieved for both quasar
components. The two-colour final records are then used to measure time delays.
The gr light curves of Q0957+561 show several prominent events and gradients,
and some of them (in the g band) lead to a time delay between components of 417
+/- 2 d (1 sigma). We do not find evidence of extrinsic variability in the
light curves of Q0957+561. We also explore the possibility of a delay between a
large event in the g band and the corresponding event in the r band. The gr
cross-correlation reveals a time lag of 4.0 +/- 2.0 d (1 sigma; the g-band
event is leading) that confirms a previous claim of the existence of a delay
between the g and r band in this lensed quasar. The time delays (between quasar
components and between optical bands) from the new records and previous ones in
similar bands indicate that most observed variations in Q0957+561 (amplitudes
of about 100 mmag and timescales of about 100 d) are very probably due to
reverberation within the gas disc around the supermassive black hole.
