Perturbative Prepotential and Monodromies in N =2 Heterotic Superstring

We discuss the prepotential describing the effective field theory of N =2 heterotic superstring models. At the one loop-level the prepotential develops logarithmic sin-gularities due to the appearance of charged massless states at particular surfaces in the moduli space of vector multiplets. These singularities modify the classical duality symmetry group which now becomes a representation of the fundamental group of the moduli space minus the singular surfaces. For the simplest two-moduli case, this fundamental group turns out to be a certain braid group and we determine the resulting full duality transformations of the prepotential, which are exact in perturbation theory.