N = 2 Instanton Calculus in Closed String Background

In this contribution we describe how to obtain instanton effects in four dimensional gauge theories by computing string scattering amplitudes in D3/D(-1) brane systems. In particular we study a system of fractional D3/D(-1) branes in a Z_2 orbifold and in a Ramond-Ramond closed string background, and show that it describes the gauge instantons of N=2 super Yang-Mills theory and their interactions with the graviphoton of N=2. Using string theory methods we compute the prepotential of the effective gauge theory exploiting the localization methods of the instanton calculus, showing that this leads to the same information given by the topological string.