  It is shown that the generating function of $\mathscr{N}=2$ topological
strings, in the heterotic weak coupling limit, is identified with the partition
function of a six-dimensional Melvin background. This background, which
corresponds to an exact CFT, realises in string theory the six-dimensional
$\varOmega$-background of Nekrasov, in the case of opposite deformation
parameters $\epsilon_1=-\epsilon_2$, thus providing the known perturbative part
of the Nekrasov partition function in the field theory limit. The analysis is
performed on both heterotic and type I strings and for the cases of ordinary
$\mathscr{N}=2$ and $\mathscr{N}=2^*$ theories.
