On Ramond-Ramond fields and K-theory

A recent paper by Moore and Witten [1] explained that Ramond-Ramond fields in type-II superstring theory have a global meaning in K-theory. In this note we amplify and generalize some points raised in that paper. In particular, we express the coupling of the Ramond-Ramond fields to D-branes in a K-theoretic framework and show that the anomaly in this coupling exactly cancels the anomaly from the fermions on the brane, both in type IIA and type IIB.