  The non-perturbative low energy effective action of N=2 SYM is studied within
a microscopic string realization via D3/D-instanton systems. The localization
deformation of instanton moduli space which has allowed the exact computation
of multi-instanton contributions is, in this setting, due to a RR graviphoton
background. The relation of deformed instanton contributions to topological
string amplitudes on CY, argued by Nekrasov, appears quite natural in this
framework. Based on arXiv:hep-th/0606013.
