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Context. The TeV BL Lac object PG 1553+113 is one of the primary candidatesfor a binary supermassive black hole system. Aims. We study the flux andspectral variability of PG 1553+113 on intra-night to long-term timescalesusing (i) BVRI data collected over 76 nights from January 2016 to August 2019involving nine optical telescopes and (ii) historical VR data (including ours)obtained for the period from 2005 to 2019. Methods. We analysed the lightcurves using various statistical tests, fitting and cross-correlationtechniques, and methods for the search for periodicity. We examined thecolour-magnitude diagrams before and after the corresponding light curves werecorrected for the long-term variations. Results. Our intra-night monitoring,supplemented with literature data, results in a low duty cycle of ~(10-18)%. InApril 2019, we recorded a flare, which marks the brightest state of PG 1553+113for the period from 2005 to 2019: R = 13.2 mag. This flare is found to show aclockwise spectral hysteresis loop on its VR colour-magnitude diagram and atime lag in the sense that the V-band variations lead the R-band ones. Weobtain estimates of the radius, the magnetic field strength, and the electronenergy that characterize the emission region related to the flare. We find amedian period of (2.21 +/- 0.04) years using the historical light curves. Inaddition, we detect a secondary period of about 210 days using the historicallight curves corrected for the long-term variations. We briefly discuss thepossible origin of this period.

Multi-band behaviour of the TeV blazar PG 1553+113 in optical range on diverse timescales