Upper panel: fiducial realization of the continuum (black) and emission line (red) light curves of a quasar (Sect. 2). The light curve corresponding to the emission line has been divided by f_BLR to show more clearly the time lag τ ~ 100 days between the continuum and the line. A negligible amount of noise has been considered to ease legibility. Bottom panel: corresponding light curves of the two lensed images in magnitude (thick blue and red lines) and the corresponding difference light curve (solid black). The dotted grey line shows the differential light curve that would be observed if the broad line was not varying, the solid green line a model of the extrinsic variations, and the thick dashed grey line is Â(t).