  By considering a (partial) topological twisting of supersymmetric Yang-Mills
compactified on a 2d space with `t Hooft magnetic flux turned on we obtain a
supersymmetric $\sigma$-model in 2 dimensions. For N=2 SYM this maps Donaldson
observables on products of two Riemann surfaces to quantum cohomology ring of
moduli space of flat connections on a Riemann surface. For N=4 SYM it maps
$S$-duality to $T$-duality for $\sigma$-models on moduli space of solutions to
Hitchin equations.
