  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.
