  We study new physical phenomena and constraints in generalized scalar--tensor
theories of gravity with $\Phi$--dependent masses. We investigate a scenario
(which can arise in string theories) with two types of $\Phi$--dependent masses
which could correspond to visible and dark matter sectors. The parameters of
this theory are constrained from post--Newtonian bounds, primordial
nucleosynthesis and the age of the Universe. We present a perfect fluid
formalism for the dark matter sector with variable masses and find an entropy
increase effect during the matter era and, in principle, a measurable effect on
the motion of the halo of spiral galaxies. For the case of string effective
theories, the constancy of gauge couplings provide new bounds which are orders
of magnitude stronger than the previous ones.
