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We have observed S5 2007+777 and 3C371 in the B and I bands for 13 and 8nights, respectively, during various observing runs in 2001, 2002 and 2004. Theobservations resulted in almost evenly sampled light curves, 6-9 hours long. Wedo not detect any flares within the observed light curves, but we do observesmall amplitude, significant variations, in both bands, on time scales of hoursand days. The average variability amplitude on time scales of minutes/hours is2.5% and 1-1.5% in the case of S5 2007+777 and 3C371, respectively. The averageamplitudes increase to 5-12% and 4-6%, respectively, on time scales of days. Wefind that the B and I band variations are highly correlated, on both short andlong time scales. During the 2004 observations, which resulted in the longestlight curves, we observe two well defined flux-decay and rising trends in thelight curves of both objects. When the flux decays, we observe significantdelays, with the B band flux decaying faster than the flux in the I band. As aresult, we also observe significant, flux related spectral variations as well.The flux-spectral relation is rather complicated, with loop-like structuresforming during the flux evolution. The presence of spectral variations implythat the observed variability is not caused by geometric effects. On the otherhand, our results are fully consistent with the hypothesis that the observedvariations are caused by perturbations which affect different regions in thejet of the sources.Comment: Accepted for publication in Astronomy and Astrophysic

B and I-band optical micro-variability observations of the BL Lac objects S5 2007+777 and 3C 371