Aspects of Dirichlet S-branes

In this Letter, we discuss some features of the Dirichlet S-brane, defined as a Dirichlet boundary condition on a time-like embedding coordinate of open strings. We analyze the Euclidean theory on the S-brane world-volume, and trace its instability to the infinite fine-tuning of the initial conditions required to produce an infinitely extended space-like defect. Using their equivalence under T-duality with D-branes with supercritical electric field, we argue that under generic perturbation, S-branes turn into D-brane/anti-D-branes. We extract the imaginary part of the cylinder amplitude, and interpret its inverse as a “decay length”, beyond which a pair of S-branes annihilates. Finally, we reconsider the boundary state of the Dirichlet S-brane and find that it is either a solution of type II string theory with imaginary R-R fields, or a solution of type II∗ with real fields. This leaves the non-BPS S-branes as potentially physical solutions of type II string theory.