BVRI photometry of BL Lacertae in 1993-1995

Istituto Astronomico, Universitá “La Sapienza", via G.M. Lancisi 29, I-00161 Roma, Italy

Send offprint request to: R. Nesci

Received: 29 May 1996
Accepted: 22 July 1996

Abstract

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.