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Stringy instanton effects in N=2 gauge theories

We study the non-perturbative effects induced by stringy instantons on N=2 SU}(N) gauge theories in four dimensions, realized on fractional D3 branes in a C^3/Z_3 orientifold. The stringy instantons, corresponding to D(-1) branes that occupy a node of the orientifold quiver diagram where no D3 brane is present, have the right content of zero-modes to produce non-perturbative terms in the four-dimensional effective action. In the SU(2) theory these terms have the same structure for all instanton numbers and yield a series of non-perturbative corrections to the prepotential. We explicitly compute these corrections up to instanton number k=5 using localization methods.