T-duality without quantum corrections

It is a well known fact that the classical (“Buscher”) transformations of T-duality do receive, in general, quantum corrections. It is interesting to check whether classical T-duality can be exact as a quantum symmetry. The natural starting point is a σ-model with N = 4 world sheet supersymmetry. Remarkably, we find that (owing to the fact that N = 4 models with torsion are not off-shell finite as quantum theories), the T-duality transformations for these models get in general quantum corrections, with the only known exception of warped products of flat submanifolds or orbifolds thereof with other geometries.