  We study a system of fractional D3 and D(-1) branes 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
supergravity. In particular, we analyze the instanton moduli space using string
theory methods and 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. We also comment
on the relation between our approach and the so-called Omega-background.
