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Abstract

This work focusses on studying the brightness variation of gravitationally lensed multiply imaged quasars. The main goal is the optimization of the relative differential photometry procedures, which are based on the difference image analysis (DIA) method. Moreover, it aims at isolating uncorrelated flux variations among the quasar images, which can be explained as due to quasar microlensing events, and at the estimation of the time delays of the observed systems from the retrieved light curves. We present V and R photometry of the gravitationally lensed quasars WFI 2033-4723, HE 0047-1756 and Q2237+0305. The analyzed data belong to the MiNDSTEp collaboration and were taken with the 1.54m Danish telescope at ESO/La Silla from 2008 to 2012. The differential photometry is based on the already published method by Alard & Lupton as implemented in the HOTPAnTS package, and additionally uses the GALFIT package for obtaining the quasar photometry. The quasar WFI 2033-4723 shows brightness variations of ~ 0.5 mag in V and R during the campaign. The two lensed components of quasar HE 0047-1756 vary by ~ 0.2 to 0.3 mag within five years. We provide for the first time an estimate of the time delay of component B with respect to A of Δt = (7.6+-1.8) days for this object. We also find evidence for a secular evolution of the magnitude difference between components A and B in both filters, which we explain as due to a long-duration microlensing event. We also find that both quasars WFI 2033-4723 and HE 0047-1756 become bluer when brighter, which is consistent with previous studies. The quasar Q2237+0305 shows impressive uncorrelated variations of the four components in both the V and R bands, with brightness variations between ~0.2 and ~1.3 mag. In particular, component D shows flux variations of ~ 1.3 mag in the V band and ~0.8 mag in the R band during the 5-year monitoring campaign, along a caustic-crossing feature of the light curve. We also find that the color of this component becomes redder by ~0.6 mag while it becomes fainter. Image C becomes brighter by ~0.7 mag between the last two monitoring seasons and this again suggests a high-magnification microlensing event.