X-ray and Swift W2 light curves of Mrk 509 (data taken from E19). The green dashed and red solid lines indicate the starting and ending points of the light curve segments that we used to compute σ_NXVobs² and σ_XVobs². The segments we used to compute the excess variance in the other optical bands are very similar to the ones indicated in the panel which shows the W2 light curve. The black points in the bottom panels of each plot show the variance measurements of each segment. The grey dashed lines and the grey area show the mean variance and its error, respectively. We draw black dotted lines at σ_NXVobs² = 0 and σ_XVobs² = 0.