FAST VARIABILITY OF BL LACERTAE AT 1 MU-M

We present the results of photometric observations of BL Lacertae at 1 mu m performed during the strong outburst that occurred in the summer of 1997. Measurements were carried out using the F99 filter of the Ar izona system. Simultaneous observations in the B and V bands were made with other telescopes. Large and fast variations were observed on the night of August 2 when the brightness of BL Lacertae changed by simil ar to 0.5 mag in about 2 h: the variation amplitude is greater at high er frequency. Some implications of these results for the origin of the fast variations are discussed. We also report the first F99 calibrati on for three reference stars in the BL Lac field.