  In an earlier paper, Alvarez, Alvarez-Gaume, Barbon and Lozano pointed out,
that the only way to "flatten" negative curvature by means of a T-duality is by
introducing an appropriate, non-constant NS-NS B-field. In this paper, we are
investigating this further and ask, whether it is possible to T-dualize AdS_d
space to flat space with some suitably chosen B. To answer this question, we
derive a relationship between the original curvature tensor and the one of the
T-dualized metric involving the B-field. It turns out that there is one
particular component, which is independent of B. By inspection of this
component, we then show, that it is not possible to dualize AdS_d to flat space
irrespective of the choice of B. Finally, we examine the extension of AdS to an
AdS_5 x S^5 geometry and propose a chain of S- and T-dualities together with an
SL(2,Z) coordinate transformation, leading to a dual D9-brane geometry.
