Abstract :
A natural SL(2,Z)-invariant generalization of the Veneziano amplitude in type IIB superstring theory is investigated. It includes certain perturbative and non-perturbative (D-instanton) contributions, and it reduces to the correct expressions in different limits. The singularities are poles in the s-t-u channels, corresponding to the exchange of particles with a mass spectrum coinciding with that of (p,q) string states. We describe the general structure of the associated perturbative corrections to the effective action.
