  Scalar field theories with appropriate potentials in Minkowski space can have
time-dependent classical solutions containing topological defects which
correspond to S-branes - i.e. branes all of whose tangential dimensions are
spacelike. It is argued that such S-branes arise in string theory as
time-dependent solutions of the worldvolume tachyon field of an unstable
D-brane or D-brane-anti-D-brane pair. Using the known coupling of the spacetime
RR fields to the worldvolume tachyon it is shown that these S-branes carry a
charge, defined as the integral of a RR field strength over a sphere
(containing a time as well as spatial dimensions) surrounding the S-brane. This
same charge is carried by SD-branes, i.e. Dirichlet branes arising from open
string worldsheet conformal field theories with a Dirichlet boundary condition
on the timelike dimension. The corresponding SD-brane boundary state is
constructed. Supergravity solutions carrying the same charges are also found
for a few cases.
