T-duality and dimensional reduction of S-brane solutions

We studied dimensional reduction and T-duality in spacelike brane solutions of 10 or 11 dimensional supergravity, including spacelike counterparts of wave and monopole solutions. Dimensional reduction is well-defined if, and only if, the solutions possess static dimensions. However, T-duality is ill-defined for some of these solutions where dilaton expectation values depend on time. This led us to conclude that supergravity solutions should be regarded as low-energy solutions of superstring theory only if dilaton expectation values are independent of time.