Topology change from (heterotic) Narain T-duality

We consider Narain T-duality on a nontrivially fibered n-torus bundle in the presence of a topologically nontrivial NS H flux. The action of the duality group on the topology and H flux of the corresponding type II and heterotic string backgrounds is determined. The topology change is specialized to the case of supersymmetric T2-fibered torsional string backgrounds with nontrivial H flux. We prove that it preserves the global tadpole condition in the total space as well as on the base of the torus fibration. We find that some of these T-dualities exchange half of the field strength of an unbroken U(1) gauge symmetry with the anti-selfdual part of the curvature of a physical circle fibration. We verify that such T-dualities indeed exchange the supersymmetry condition for the circle bundle with that of the gauge bundle.