We study twisted N=2 superconformal gauge theory on a product of two Riemann
surfaces Sigma and C. The twisted theory is topological along C and holomorphic
along Sigma and does not depend on the gauge coupling or theta-angle. Upon
Kaluza-Klein reduction along Sigma, it becomes equivalent to a topological
B-model on C whose target is the moduli space MV of nonabelian vortex equations
on Sigma. The N=2 S-duality conjecture implies that the duality group acts by
autoequivalences on the derived category of MV. This statement can be regarded
as an N=2 counterpart of the geometric Langlands duality. We show that the
twisted theory admits Wilson-'t Hooft loop operators labelled by both electric
and magnetic weights. Correlators of these loop operators depend
holomorphically on coordinates and are independent of the gauge coupling. Thus
the twisted theory provides a convenient framework for studying the Operator
Product Expansion of general Wilson-'t Hooft loop operators.Comment: 50 pages, latex. v2: an erroneous statement about an analog of the
Hitchin fibration has been fixe
