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The results of optical photometry of BL Lac over a time interval of two years in the B, V, R, I l- ters 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 consid- erations on the physical conditions driving the appearance of emission lines are made.

BVRI photometry of BL Lacertae in 1993-1995