Modelling of the long-term flux ratio variations from realistic microlensing simulations. Left panel: magnification map, convolved by a thin-disk light profile with R₀ = 7.9 × 10¹⁴ cm (0.3 light-days). The five best-fit trajectories are shown in colours. Right panel: observed flux ratios F_B/F_A of the two lensed images. The observations are averaged by season in order to fit only the long-term variations, attributed to the displacement of the quasar through the micro-caustic network. The five best-fit trajectories are shown as dashed lines. The horizontal dotted line corresponds to the flux ratio expected from the macro lens model. The legend indicates the total transverse velocity, V, corresponding to the selected trajectory as well as the associated reduced χ² of the fit.