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We extend the gr-band time coverage of the gravitationally lensed doublequasar Q0957+561. New gr light curves permit us to detect significant intrinsicfluctuations, to determine new time delays, and thus to gain perspective on themechanism of intrinsic variability in Q0957+561. We use new optical frames ofQ0957+561 in the g and r passbands from January 2005 to July 2007. These framesare part of an ongoing long-term monitoring with the Liverpool robotictelescope. We also introduce two photometric pipelines that are applied to thenew gr frames of Q0957+561. The transformation pipeline incorporateszero-point, colour, and inhomogeneity corrections to the instrumentalmagnitudes, so final photometry to the 1-2% level is achieved for both quasarcomponents. 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 thelight curves of Q0957+561. We also explore the possibility of a delay between alarge event in the g band and the corresponding event in the r band. The grcross-correlation reveals a time lag of 4.0 +/- 2.0 d (1 sigma; the g-bandevent is leading) that confirms a previous claim of the existence of a delaybetween the g and r band in this lensed quasar. The time delays (between quasarcomponents and between optical bands) from the new records and previous ones insimilar bands indicate that most observed variations in Q0957+561 (amplitudesof about 100 mmag and timescales of about 100 d) are very probably due toreverberation within the gas disc around the supermassive black hole.

New two-colour light curves of Q0957+561: time delays and the origin of intrinsic variations