BVI Microvariability in BL LAC during the summer 1997 outburst

BL Lac has been observed during the outburst occurred in the 1997 summer (Noble et al. 1997; Bloom et al. 1997) in three optical bands (BVI), with the aim of studying the intranight colour variations on very short timescales. Due to the high luminosity of the source we could perform quasi-simultaneous measurements in the three bands with a resolving time of about 5 minutes using one single telescope. We have detected several microvariability events occurring on timescales ranging from 30 min up to 3.5 hours. The magnitude changes had different amplitudes in the three bands being larger for shorter wavelenghts, and the source showed a marked trend to became bluer when brigther. There was no evidence of quasi-periodic oscillations in our multicolour light curves.