AN ANSATZ FOR A NONPERTURBATIVE 4-GRAVITON AMPLITUDE IN TYPE IIB SUPERSTRING THEORY

A natural SL(2,Z)-invariant generalization of the Veneziano amplitude in type IIB superstring theory is investigated. It includes certain pe rturbative and non-perturbative (D-instanton) contributions, and it re duces to the correct expressions in different limits, The singularitie s are poles in the s-t-u channels, corresponding to the exchange of pa rticles with a mass spectrum coinciding with that of (p,q) string stat es. We describe the general structure of the associated perturbative c orrections to the effective action. (C) 1998 Elsevier Science B.V.