D-effects in Toroidally Compactified Type II String Theory

We review exact results obtained for R couplings in maximally supersymmetric type II string theories. These couplings offer a privileged scene to understand the rules of semiclassical calculus in string theory. Upon expansion in weak string coupling, they reveal an infinite sum of non-perturbative e−1/g effects that can be imputed to euclidean D-branes wrapped on cycles of the compactification manifolds. They also shed light on the relation between Dp-branes and D-(p-2)branes, D-strings and (p, q) strings, instanton sums and soliton loops. The latter interpretation takes over in D ≤ 6 in order to account for the e−1/g 2 effects, still mysterious from the point of view of instanton calculus.