Stringy instanton corrections toN = 2 gauge couplings

We discuss a string model where a conformal four-dimensionalN = 2 gauge theory receives corrections to its gauge kinetic functions from \stringy" instantons. These contributions are explicitly evaluated by exploiting the localization properties of the integral over the stringy instanton moduli space. The model we consider corresponds to a setup with D7/D3-branes in type I 0 theory compactied on T4=Z2T 2, and possesses a perturbatively computable heterotic dual. In the heteoric side the corrections to the quadratic gauge couplings are provided by a 1-loop threshold computation and, under the duality map, match precisely the rst few stringy instanton eects in the type I 0 setup. This agreement represents a very non-trivial test of our approach to the exotic instanton calculus.