The Astronomical Journal
BL Lac was observed in the V and R bands on 11 nights in 1997 July during a major optical outburst. It varied continuously, exhibiting nightly variations ranging from 0.1 to 0.6 mag. Over the course of the 11 nights, BL Lac was seen to vary by 1.8 R magnitudes. Color variations were also observed, with BL Lac becoming bluer as it brightened. The influence of variable seeing conditions on the observed variations was investigated. No correlation was found between the observed microvariability and the local seeing conditions.
Clements, Sandra D. and Carini, Michael T., "Multiband Microvariability Observations Of BL Lacertae During The Outburst Of 1997" (2001). Aerospace, Physics, and Space Science Faculty Publications. 281.