Colour excess E_g,r versusA_V,DL for the QSOs with U_min = 0.6. Colour corresponds to the density of points (see Fig. 5). The straight line corresponds to the best fit for all the QSOs. For eachA_V,DL, the black curve corresponds to the mean E_g,r for the QSOs in an interval with a half-width δA_V,DL = 0.01. Even though the QSOs show significant scatter, the E_g,r versusA_V,DL relationship is very linear; the mean E_g,r for eachA_V,DL curve does not show significant departures from the straight line.