Extinction curve fit result. Top panel: empirical average extinction curve, γ(λ), compared to the best fit by a Cardelli et al. (1989) law, which gives an R_V = 2.6. The shaded blue area represent a ±0.5 range on R_V, in order to illustrate the typical range of value that are measured for SN Ia in the literature. The black dot on this curve indicates the wavelength used on the lower panel graph. Lower panel: residuals after correction for the intrinsic behavior at 4012 Å as a function of A_λ0. Each dot corresponds to a single SN Ia. The γ_{4012 Å} slope between M and A_λ0 is indicated by the red continuous line. Dashed represent the Cardelli et al. (1989) law for R_V = 2.6. The shaded blue area represent a ±0.5 range on R_V, in order to illustrate the typical range of value that are measured for SN Ia in the literature.