T-DUALITY WITHOUT QUANTUM CORRECTIONS

It is a well known fact that the classical (''Buscher'') transformatio ns of T-duality do receive, in general, quantum corrections. It is int eresting to check whether classical T-duality can be exact as a quantu m 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 quant um corrections, with the only known exception of warped products of fl at submanifolds or orbifolds thereof with other geometries. (C) 1997 E lsevier Science B.V.