  To go into the details about the variability of the double quasar SBS
0909+532, we designed a monitoring programme with the 2 m Liverpool Robotic
Telescope in the r Sloan filter, spanning 1.5 years from 2005 January to 2006
June. The r-band light curves of the A and B components, several
cross-correlation techniques and a large number of simulations (synthetic light
curves) lead to a robust delay of 49 +/- 6 days (1-sigma interval) that agrees
with our previous results (the B component is leading). Once the time delay and
the magnitude offset are known, the magnitude- and time-shifted light curve of
image A is subtracted from the light curve of image B. This difference light
curve of SBS 0909+532 is consistent with zero, so any possible extrinsic signal
must be very weak, i.e., the observed variability in A and B is basically due
to observational noise and intrinsic signal. We then make the combined light
curve and analyse its statistical properties (structure functions). The
structure function of the intrinsic luminosity is fitted to predictions of
simple models of two physical scenarios: accretion disc instabilities and
nuclear starbursts. Although no simple model is able to accurately reproduce
the observed trend, symmetric triangular flares in an accretion disc seems to
be the best option to account for it.
