Open Superstring Theory and Superalgebra of the Brane Antibrane System
  Open superstring theory is formulated in terms of a nondegenerate supertranslation algebra. A supercharge for a tachyonic superstring can be also defined classically by taking into account the leakage of the supercurrent which is compensated by fermionic and bosonic auxiliary fields. The anticommutator of two supercharges of the tachyonic superstring does not contain the zero eigenvalue and so this string is not a BPS state. Brane-antibrane annihilation scenarios are described by these superalgebras defined on the sum of world-volumes of a D-brane, an anti-D-brane and a tachyonic superstring. 
Machiko Hatsuda and Makoto Sakaguchi