ΔBC = BC_AV − BC_AV = 0 as a function of T_eff for Gaia G band (left panel), Gaia G_BP − G_RP colour (central panel), and the WFC3/UVIS F218W filter (right panel). The grey open circles denote the models with fixed extinction coefficients at the filter centres, which are connected by the solid lines. The dashed lines indicate a linear extrapolation of the open circles of A_V = 0 and A_V = 1 mag, for the same T_eff. The ATLAS9 models are used in this figure, but the PHOENIX models give consistent results.