Effects of D-instantons

Scattering of fundamental states of type IIB supergravity and superstring theory is discussed at low orders in perturbation theory in the background of a D-instanton. The integration over fermionic zero-modes in both the low energy supergravity and in the string theory leads to explicit non-perturbative terms in the effective action. These include a single instanton correction to the known tree-level and one-loop R4 interactions. The 'spectrum' of multiply charged D-instantons is deduced by T-duality in nine dimensions from multiply wound world-lines of marginally bound D-particles. This, and other clues, lead to a conjectured SL(2,Z) completion of the R4 terms which suggests that they are not renormalized by perturbative corrections in the zero-instanton sector beyond one-loop. The string theory unit-charged D-instanton gives rise to point-like effects in fixed-angle scattering, raising unresolved issues concerning distance scales in superstring theory. (orig.)