ASTRONOMY AND ASTROPHYSICS Unusually rapid variability of the GRB000301C optical afterglow

We present BV RI light curves of the afterglow of GRB000301C, one of the brightest ever detected at a day time scale interval after GRB trigger. The monitoring started 1. 5 days after the GRB and ended one month later. Inspection of the extremely well sampled R band light curve and compari- son with BV I data has revealed complex behavior, with a long term flux decrease and various short time scale features supe r- imposed. These features are uncommon among other observed afterglows, and might trace either intrinsic variability w ithin the relativistic shock (re-acceleration and re-energizat ion) or inhomogeneities in the medium in which the shock propagates.