Non-perturbative corrections in N=2 strings
  We investigate the non-perturbative equivalence of some heterotic/type IIA dual pairs with N=2 supersymmetry. We compute R2-like corrections, both on the type IIA and on the heterotic side. The coincidence of their perturbative part provides a test of duality. The type IIA result is then used to predict the full, non-perturbative correction to the heterotic effective action. We determine the instanton numbers and the Olive-Montonen duality groups. 
Andrea Gregori