The structure of N=2 multi-instanton and string-loop corrections in   toroidal orbifold models
  We summarize the structure of N=2 multi-instanton and string loop corrections to the K\"ahler potential and the gauge kinetic function in type I toroidal orbifold models. For that aim, we exploit perturbative calculations of the physical gauge couplings on the heterotic dual, whenever such a perturbative description is available. 
Pablo G. Camara