Fivebrane instantons and higher derivative couplings in type I theory Original Research Article

We express the infinite sum of D5-brane instanton corrections to R2 couplings in N=4 type I string vacua, in terms of an elliptic index counting 12-BPS excitations in the effective Sp(N) brane theory. We compute the index explicitly in the infrared, where the effective theory is argued to flow to an orbifold CFT. The form of the instanton sum agrees completely with the predicted formula from a dual one-loop computation in type IIA theory on K3×T2. The proposed CFT provides a proper description of the whole spectrum of masses, charges and multiplicities for 12- and 14-BPS states, associated to bound states of D5-branes and KK momenta. These results are applied to show how fivebrane instanton sums, entering higher derivative couplings which are sensitive to 14-BPS contributions, also match the perturbative results in the dual type IIA theory.