  String effective theories contain a dilaton scalar field which couples to
gravity, matter and radiation. In general, particle masses will have different
dilaton couplings. We can always choose a conformal frame in which baryons have
constant masses while (non--baryonic) dark matter have variable masses, in the
context of a scalar--tensor gravity theory. We are interested in the
phenomenology of this scenario. Dark matter with variable masses could have a
measurable effect on the dynamical motion of the halo of spiral galaxies, which
may affect cold dark matter models of galaxy formation. As a consequence of
variable masses, the energy--momentum tensor is not conserved; there is a
dissipative effect, due to the dilaton coupling, associated with a ``dark
entropy" production. In particular, if axions had variable masses they could be
diluted away, thus opening the ``axion window". Assuming that dark matter with
variable masses dominates the cosmological evolution during the matter era, it
will affect the primordial nucleosynthesis predictions on the abundances of
light elements. Furthermore, the dilaton also couples to radiation in the form
of a variable gauge coupling. Experimental bounds will constrain the parameters
of this model.
