Non-BPS D-branes and enhanced symmetry in an asymmetric orbifold

In this paper properties of D branes in a nine dimensional asymmetric orbifold are discussed, using a $(-1)^{F_L}σ_{1/2}$ projection, where $F_L$ is the leftmoving space-time fermion number and $σ_{1/2}$ is a freely acting shift of order two. There are two types of non BPS D branes, which are stable at $R>2$ and $R<2$ respectively. At R=2 there is a perturbative enhancement of gauge symmetry and the two types of branes are related by a global bulk symmetry transformation. At this point in the moduli space the associated boundary states are constructed using a free fermion representation of the theory. Some aspects of the enhancement of gauge symmetry in the S-dual type $\tilde I$ theory are discussed.

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