Top: spectral energy distribution of GRB 130925A during the optical/NIR peak as observed with GROND. The emission of the host galaxy is subtracted. There is no detection in the g′r′-bands above the host emission. The curvature in the SED is due to the strong dust extinction of A_V = 5.0 ± 0.7 mag. The inset shows contours of constant χ_red for the intrinsic spectral slope as a function of reddening (SMC reddening law A_V = 2.93 × E(B − V)) for the peak. Bottom: evolution of the spectral slope (spectral index β which relates to the photon index via β + 1) of the optical/NIR afterglow with time with fixed A_V. For most temporal slices outside the optical/NIR peak, the spectral energy distribution is defined only through JHK_s measurements, and thus the error is relatively large. The horizontal bars visualize the average slope for the peak (right) and the pre-peak (left) period.