We study the non-perturbative effects induced by stringy instantons on ????=2 SU(N) gauge theories in four dimensions, realized on fractional D3 branes in a ℂ3/ℤ3 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.

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