Abstract We monitored BL Lacertae in the B, V , R and I bands for 14 nights during the period of 2016– 2018. The source showed significant intra-day variability on 12 nights. We performed color-magnitude analysis and found that the source exhibited bluer-when-brighter chromatism. This bluer-when-brighter behavior is at least partly caused by the larger variation amplitude at shorter wavelengths. The variations at different wavelengths are correlated well and show no inter-band time lag.
Keywords galaxies: active — BL Lacertae objects: individual: BL Lacertae — galaxies: photometry
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