Multi-instanton and string loop corrections in toroidal orbifold models

We analyze N = 2 (perturbative and non-perturbative) corrections to the effective-theory in type I orbifold models where a dual heterotic description is available. These corrections may play an important role in phenomenological scenarios. More precisely, we consider two particular compactifications: the Bianchi-Sagnotti-Gimon-Polchinski orbifold and a freely-acting Z(2) x Z(2) orbifold with N = 1 supersymmetry and gauge group SO(q) x SO(32 - q). By exploiting perturbative calculations of the physical gauge couplings on the heterotic side, we obtain multi-instanton and one-loop string corrections to the Kahler potential and the gauge kinetic function for these models. The non-perturbative corrections appear as sums over relevant Hecke operators, whereas the one-loop correction to the Kahler potential matches the expression proposed in [1, 2]. We argue that these corrections are universal in a given class of models where target-space modular invariance (or a subgroup of it) holds.