D-instanton corrections as (p,q)-string effects and non-renormalization theorems

We discuss higher derivative interactions in the type IIB superstring in ten dimensions. From the fundamental string point of view, the non-perturbative corrections are due to D-instantons. We argue that they can alternatively be understood as arising from (p,q)-strings. We derive a non-renormalization theorem for eight-derivative bosonic interactions, which states that terms involving 2r R-R fields occur at r and r+1 loops only. By using the SL(2,Z) symmetry of M-theory on T^2, we show that in order for the possible R^{3m+1} (m=1,2,...) interactions in M-theory to have a consistent perturbative expansion in nine dimensions, m must be odd. Thus, only R^{6N+4} (N=0,1,...) terms can be present in M-theory and their string theory counterparts arise at N and 2N+1 loops. Finally, we treat an example of fermionic term.