DYNAMICAL MASS MATRICES FROM EFFECTIVE SUPERSTRING THEORIES

We analyze the general structure of the fermion mass matrices in effec tive superstrings. They are generically given at low energy by non-tri vial functions of the gauge singlet moduli fields. Interesting structu res appear in particular if they are homogeneous functions of zero deg ree in the moduli. In this case we find Yukawa matrices very similar t o the ones obtained by imposing a U(1) family symmetry to reproduce th e observed hierarchy of masses and mixing angles. The role of the U(1) symmetry is played here by the modular symmetry. We discuss the advan tage of such an approach regarding flavor changing neutral currents. E xplicit orbifold examples are given where realistic quark mass matrice s can be obtained. Finally, a complete scenario is proposed which gene rates the observed hierarchies in a dynamical way,