  We have observed S5 2007+777 and 3C371 in the B and I bands for 13 and 8
nights, respectively, during various observing runs in 2001, 2002 and 2004. The
observations resulted in almost evenly sampled light curves, 6-9 hours long. We
do not detect any flares within the observed light curves, but we do observe
small amplitude, significant variations, in both bands, on time scales of hours
and days. The average variability amplitude on time scales of minutes/hours is
2.5% and 1-1.5% in the case of S5 2007+777 and 3C371, respectively. The average
amplitudes increase to 5-12% and 4-6%, respectively, on time scales of days. We
find that the B and I band variations are highly correlated, on both short and
long time scales. During the 2004 observations, which resulted in the longest
light curves, we observe two well defined flux-decay and rising trends in the
light curves of both objects. When the flux decays, we observe significant
delays, with the B band flux decaying faster than the flux in the I band. As a
result, we also observe significant, flux related spectral variations as well.
The flux-spectral relation is rather complicated, with loop-like structures
forming during the flux evolution. The presence of spectral variations imply
that the observed variability is not caused by geometric effects. On the other
hand, our results are fully consistent with the hypothesis that the observed
variations are caused by perturbations which affect different regions in the
jet of the sources.
