BVI MICROVARIABILITY IN BL LACERTAE DURING THE SUMMER 1997 OUTBURST

BL Lac has been observed during the outburst which occurred during the 1997 summer (Noble et al. 1997; Bloom et al. 1997) in three optical b ands (BVIC), 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 ha ve detected several microvariability events occurring on timescales ra nging from 30 min up to 3.5 hours. The magnitude changes had different amplitudes in the three bands being larger for shorter wavelengths, a nd the source showed a marked trend to become bluer when brigther. The re was no evidence of quasi-periodic oscillations in our multicolour l ight curves.