Colors and luminosities of the optical afterglows of the gamma-ray bursts

Results of the study of the color indices and luminosities of 17 optical af terglows (OAs) of GRBs are presented. We show that the color variations dur ing the decline of OAs (except for GRB000131) are relatively small during t T-o < 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)(o) = 0.40 +/- 0.13, (R - I)(o) = 0.46 +/- 0.18, (B - V)(o) = 0.47 +/- 0.17. The color evolution of the OAs is neglig ible although their brightness declines by several magnitudes during the co nsidered time interval. Such a strong concentration of the color indices al so suggests that the intrinsic reddening (inside their host galaxies) must be quite similar and relatively small for all these events. The absolute br ightness of OAs in the observer frame, corrected for the host galaxy, lies within M-Ro = -26.5 to -22.2 for (t - T-o)(rest) = 0.25 days. This spread o f M-Ro is not significantly influenced by the shifts of <lambda>, caused by the different redshift z of the respective OAs. The general decline rate o f the OA sample considered here seems to be independent of the absolute opt ical brightness of the OA, measured at some t - T-o identical for all OAs, and the light curves of all events are almost parallel, when corrected for the redshift-induced time dilation.