  We discuss the prepotential describing the effective field theory of N=2
heterotic superstring models. At the one loop-level the prepotential develops
logarithmic singularities 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.
