Calculable e^{-1/λ} Effects

Abstract

We identify and evaluate a class of physical amplitudes in four-dimensional N=4 superstring theory, which receive, in the weak coupling limit, contributions of order e^{-1/λ}, where λis the type II superstring coupling constant. They correspond to four-derivative \Ftilde_1 interaction terms involving the universal type II dilaton supermultiplet. The exact result, obtained by means of a one-loop computation in the dual heterotic theory compactified on T^6, is compared with the perturbation theory on the type II side, and the e^{-1/λ} contributions are associated to non-perturbative effects of Euclidean solitons (D-branes) wrapped on K3 x T^2. The ten-dimensional decompactification limit on the type IIB side validates the recent conjecture for the D-instanton-induced R^4 couplings.

LaTex, 24 pages; typos corrected, 4 references added, D-brane interpretation improved at end of Section 5. Final version