Orientifolds, RR torsion, and K-theory

We analyze the role of RR fluxes in orientifold backgrounds from the point of view of K-theory, and demonstrate some physical implications of describi ng these fluxes in K-theory rather than cohomology. In particular, we show that certain fractional shifts in RR charge quantization due to discrete RR fluxes are naturally explained in K-theory. We also show that some orienti fold backgrounds, which are considered distinct in the cohomology classific ation, become equivalent in the K-theory description, while others become u nphysical.