Abstract
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 
= 2 super Yang-Mills theory
and their interactions with the graviphoton of = 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 Ω-background.
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