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 intraday variability on 12 nights. We performed colour-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 wavelength. The variations at different wavelengths are well correlated and show no inter-band time lag.
@article{arxiv.2108.04699,
title = {Intra-day variability of BL Lacertae from 2016 to 2018},
author = {Tian Li and Jianghua Wu and Nankun Meng and Yan Dai and Xiaoyuan Zhang},
journal= {arXiv preprint arXiv:2108.04699},
year = {2021}
}