  We studied the optical afterglows of the 24 pre-Swift Gamma-Ray Bursts with
known spectroscopic redshift and published estimates of the optical extinction
in the source frame. We find an unexpected clustering of the optical afterglow
luminosities measured 12 hours (source frame time) after the trigger. For 21
out of 24 bursts, the distribution of the optical luminosities is narrower than
the distribution of the X-ray luminosities, and even narrower than the
distribution of the ratio between the monochromatic optical luminosities and
the total isotropic emitted prompt energy. Three bursts stand apart from the
distribution of the other sources, being underluminous by a factor ~15. We
compare this result with the somewhat analogous result concerning the
luminosity of the X-ray afterglows studied by Gendre & Boer. For all our GRBs
we construct the optical to X-ray spectral energy distribution. For all but a
minority of them, the optical and the X-ray emissions are consistent with being
produced by the same radiation process. We discuss our results in the framework
of the "standard" external shock synchrotron model. Finally, we consider the
behavior of the first GRBs of known redshifts detected by Swift. We find that
these Swift GRBs entirely confirm our findings.
