Return of the Torsion D-Branes

We study D-branes on Calabi-Yau manifolds, carrying charges which are torsion elements of the K-theory. Interesting physics ensues when we follow these branes into nongeometrical phases of the compactification. On the level of K-theory, we determine the monodromies of the group of charges as we circle singular loci in the closed string moduli space. Going beyond K-theory, we discuss the stability of torsion D-branes as a function of the K\"ahler moduli. When the fundamental group of the Calabi-Yau is nonabelian, we find evidence for new threshold bound states of BPS branes. In a two-parameter example, we compare our results with computations in the Gepner model. Our study of the torsion D-branes in the compactification of [FHSV] sheds light on the physics of that model. In particular, we develop a proposal for the group of allowed D-brane charges in the presence of discrete RR fluxes