Stringy instantons and dualities

We discuss non‐perturbative corrections to the gauge kinetic functions in a four‐dimensional N = 2 gauge theory realized with a system of D7/D3‐branes in a compactification of type I′ theory on T4Z2 × Z2. The non‐perturbative contributions arise when D(–1) branes, corresponding to stringy instantons, are added to the system; such contributions can be explicitly evaluated using localization techniques and precisely match the results predicted by the heterotic/type I′ duality. This agreement represents a very non‐trivial test of the stringy multi‐instanton calculus.