2001A&A...377..450S

Result. of the study of the color indices and luminosities of 17 optical afterglows (OAs) of GRBs are presented. We show that the color variations during the decline of OAs (except for GRB000131) are relatively small during t-T₀<10 days and allow a comparison among them, even for the less densely sampled OAs. The colors in the observer frame, corrected for the Galactic reddening, concentrate at (V-R)₀=0.40±0.13, (R-I)₀=0.46±0.18, (B-V)₀=0.47±0.17. The color evolution of the OAs is negligible although their brightness declines by several magnitudes during the considered time interval. Such a strong concentration of the color indices also suggests that the intrinsic reddening (inside their host galaxies) must be quite similar and relatively small for all these events. The absolute brightness of OAs in the observer frame, corrected for the host galaxy, lies within M_R0=-26.5 to -22.2 for (t-T₀)_rest=0.25 days. This spread of M_R0 is not significantly influenced by the shifts of λ, caused by the different redshift z of the respective OAs. The general decline rate of the OA sample considered here seems to be independent of the absolute optical brightness of the OA, measured at some t-T₀ identical for all OAs, and the light curves of all events are almost parallel, when corrected for the redshift-induced time dilation.