Left: mean R_{S /V} as a function of the mean T_dust, each plotted with the standard deviation, in bins of equal number. Two versions of the Planck data have been used: with (black) and without (red) correction for leakage of intensity into polarization. Middle: the same, but for R_P/p. Right: the same, but for I_S/A_V. The blue curves, motivated in the right panel, show the expected response of the ratios to an increase in T_dust, according to a simple model in which the subset of grains that are polarizing had the same temperature as characterized the total emission (see text).