Illustration of the regression difference technique on light curves of HE 0435-1223 (Courbin et al. 2011). For clarity, only the light curves of images A (red) and B (blue) are shown. The Gaussian process regressions are shown as red and blue continuous mean functions and 1σ envelopes. Curve B has been shifted in time with respect to A to minimize the weighted total variation of the A − B difference curve, shown in black. The brown difference curve was obtained by deliberately shifting B by 15 days with respect to its optimal position. The curves have been arbitrarily offset in magnitudes for display purposes.