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 sigma-model with N=4 world sheet supersymmetry. Remarkably, we find thai (owing to the fact that N=4 models with torsion are riot 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. (C) 1997 Elsevier Science B.V.