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