  The allowed mass windows for a cosmic background of relic dilatons are
estimated in the context of the pre-big bang scenario. The dilatons are
produced from the quantum fluctuations of the vacuum, and the extension of the
windows is controlled by the string mass scale. The possible relaxation of
phenomenological bounds due to an intermediate stage of reheating is discussed.
Even without such a relaxation, the allowed range of masses includes a light
sector in which the dilatons are not yet decayed, and could provide the
dominant contribution to the present large scale density.
