  Nekrasov's partition function is defined on a flat bundle of R^4 over S^1
called the Omega background. When the fibration is self-dual, the partition
function is known to be equal to the topological string partition function,
which computes scattering amplitudes of self-dual gravitons and graviphotons in
type II superstring compactified on a Calabi-Yau manifold. We propose a
generalization of this correspondence when the fibration is not necessarily
self-dual.
