Calculable e−1λ effects Original Research Article

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−1λ, where λ is the type II superstring coupling constant. They correspond to four-derivative F1 interaction terms involving the universal type II dilaton supermultiplet. The exact results, obtained by means of a one-loop computation in the dual heterotic theory compactified on ⊤6, is compared with the perturbation theory on the type II side, and the e−1λ contributions are associated to non-perturbative effects of Euclidean solitons (D-branes) wrapped on K3 × ⊤2. The ten-dimensional decompactification limit on the type IIB side validates the recent conjecture for the D-instanton-induced R4 couplings.