We monitored BL Lacertae simultaneously in the optical B, V, R and I bands for 13 nights during the period 2012-2016. The variations were well correlated in all bands and the source showed significant intraday variability (IDV). We also studied its optical flux and colour behaviour, and searched for inter-band time lags. A strong bluer-whenbrighter chromatism was found on the intra-night time-scale. The spectral changes are not sensitive to the host galaxy contribution. Cross-correlation analysis revealed possible time delay of about 10 min between variations in the V and R bands. We interpreted the observed flares in terms of the model consisting of individual synchrotron pulses.

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We perform quasi-simultaneous optical multi-band monitoring of BL Lac object S5 0716+714 on seven nights from 2013 to 2016. Intra-day variability (IDV) is found on all seven nights. The source was faintest on JD 2456322 with 14.15 mags and brightest on JD 2457437 with 12.51 mags in the R band. The maximum intra-day variation we observed is 0.15 mags in the B band on JD 2456322. Both bluer-whenbrighter and achromatic spectral behaviours were observed on the intra-day timescale. On the longer-term scale, the object exhibited a mild bluer-when-brighter behaviour between the B and R bands. We estimate the inter-band lags using two independent methods. The variation in the B band was observed to lag that in the I band by about 15 minutes on JD 2457315. We compare this lag with one reported previously and discussed the origin of these lags.

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.

We monitored the BL Lac object S5 0716+714 in four optical wavebands with four telescopes on 11 nights from 2012 to 2017. The object was active during most of those nights. It was brightest on 2012 November 07 with 11.720 mags in the I band and reached the faintest state on 2016 January 22 with 15.770 mags in the R band corresponding to ∼15.230 mags in the I band. Intraday variations in all bands were found on 6 days, while in partial bands on 3 days. Strong bluer-when-brighter chromatism was observed on 8 nights. Correlation analyses revealed inter-band time delays from 4 to 40 minutes in the intraday variability on 5 nights. Both hard and soft lags were observed in this object, indicating complicated emission process in the jet. The analysis on the interband time delays indicated that the radiative cooling may dominate the optical time lags in S5 0716+714 occasionally, but can not be invoked as the sole explanation.