arXiv:hep-th/9707222v2 21 Oct 1997 CERN–TH/97-179 CPTH–S550.0797 hep-th/9707222 Calculable e −1/λ Effects ⋆ I. Antoniadis a,b , B. Pioline a,b and T.R. Taylor b,c a Centre de Physique Th´ eorique, Ecole Polytechnique, † F-91128 Palaiseau, France b Theory Division, CERN, 1211 Geneva 23, Switzerland c Department of Physics, Northeastern University, Boston, MA 02115, U.S.A. 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 or- der e −1/λ , where λ is the type II superstring coupling constant. They correspond to four-derivative  F 1 interaction terms involving the universal type II dilaton super- multiplet. 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 ×T 2 . The ten-dimensional decompactification limit on the type IIB side validates the recent conjecture for the D-instanton–induced R 4 couplings. ⋆ Research supported in part by the National Science Foundation under grant PHY–96–02074, and in part by the EEC under the TMR contract ERBFMRX-CT96-0090. † Laboratoire Propre du CNRS UPR A.0014. CERN–TH/97-179 July 1997