  We studied the behaviour of the optical afterglow lightcurves of a sample of
24 Gamma--Ray Bursts (GRBs) with known redshift and published estimates of the
optical extinction in the source frame, detected before the SWIFT satellite
launch. We found an unexpected clustering of the optical luminosities at 12
hours in the source frame. The distribution of the optical luminosities is
narrower than the distribution of X-ray luminosities at the same time. Few (3)
bursts stand apart from the main optical distribution, being fainter by a
factor of about 15. We also analysed the optical luminosities of the SWIFT
burst with known redshift finding that the luminosity distribution is similar
to the pre SWIFT GRBs one, even if they have a different mean redshift. These
results can suggest the existence of a family of intrinsically optically
under--luminous dark GRBs.
