Stable non-BPS D-branes in type I string theory

We use thr boundary state formalism to study, from the closed string point of view, superpositions of branes and anti-branes which are relevant in som e non-perturbative string dualities. Treating the tachyon instability of th ese systems as proposed by Sen, we show how to incorporate the effects of t he tachyon condensation directly in the boundary state. In this way we mana ge to show explicitly that the D1-anti-D1 pair of Type I is a stable non-BP S D-particle, and compute its mass. We also generalize this construction to describe other non-BPS D-branes of Type I. By requiring the absence of tac hyons in the open string spectrum, we find which configurations are stable and compute their tensions. Our classification is in complete agreement wit h the results recently obtained using the K-theory of space-time. (C) 2000 Elsevier Science B.V. All rights reserved.