  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 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.
