In string theory

String theory

String theory is an active research framework in particle physics that attempts to reconcile quantum mechanics and general relativity. It is a contender for a theory of everything , a manner of describing the known fundamental forces and matter in a mathematically complete system...

, an S-brane is a hypothetical and controversial counterpart of a D-brane

D-brane

In string theory, D-branes are a class of extended objects upon which open strings can end with Dirichlet boundary conditions, after which they are named. D-branes were discovered by Dai, Leigh and Polchinski, and independently by Hořava in 1989...

, which, unlike a D-brane, is localized in time. Depending on the context the "S" stands for "Strominger", "Sen", or "Space-like". The S-brane was originally proposed by Andrew Strominger

Andrew Strominger

Andrew Eben Strominger is an American theoretical physicist who works on string theory and son of Jack L. Strominger. He is currently a professor at Harvard University and a senior fellow at the Society of Fellows...

 in his speculative paper with Michael Gutperle, and another version of S-branes was studied by Ashoke Sen

Ashoke Sen

Ashoke Sen , FRS, is an Indian theoretical physicist. He has made a number of major original contributions to the subject of string theory, including his landmark paper on strong-weak coupling duality or S-duality, which was influential in changing the course of research in the field...

. They are thought to be extended in the space-like/temporal directions only, so that they exist at only one point in time, but are otherwise analogous to p-branes.