  The luminosities of the optical afterglows of Gamma Ray Bursts, 12 hours
(rest frame time) after the trigger, show a surprising clustering, with a
minority of events being at a significant smaller luminosity. If real, this
dichotomy would be a crucial clue to understand the nature of optically dark
afterglows, i.e. bursts that are detected in the X-ray band, but not in the
optical. We investigate this issue by studying bursts of the pre-Swift era,
both detected and undetected in the optical. The limiting magnitudes of the
undetected ones are used to construct the probability that a generic bursts is
observed down to a given magnitude limit. Then, by simulating a large number of
bursts with pre-assigned characteristics, we can compare the properties of the
observed optical luminosity distribution with the simulated one. Our results
suggest that the hints of bimodality present in the observed distribution
reflects a real bimodality: either the optical luminosity distributions of
bursts is intrinsically bimodal, or there exists a population of bursts with a
quite significant grey absorption, i.e. wavelength independent extinction. This
population of intrinsically weak or grey-absorbed events can be associated to
dark bursts.
