A&A Supplement series, Vol. 122, April II 1997, 267-270
Received May 29; accepted July 22, 1996
M. Maesano, F. Montagni, E. Massaro, and R. Nesci
Send offprint request: R. Nesci at astrm2.rm.astro.it
Istituto Astronomico, Universitá ``La Sapienza", via G.M. Lancisi 29, I-00161 Roma, Italy
The results of optical photometry of BL Lac over a time interval of two years in the B, V, R, I filters are presented. The source is found for most of the time in a low state. The mean colour indexes are similar to those observed in higher luminosity states, however a flattening of the spectral slope was present during three small amplitude outbursts. A brief comparison with the observations available in the literature and some considerations on the physical conditions driving the appearance of emission lines are made.
keywords: BL Lacertae objects: general -- BL Lacertae: individual: BL Lac