D-Branes in Orientifolds and Orbifolds and

A classification of D-branes in Type IIB Op − orientifolds and orbifolds in terms of Real and equivariant KK-groups is given. We classify D-branes intersecting orientifold planes from which are recovered some special limits as the spectrum for D-branes on top of Type I Op − orientifold and the bivariant classification of Type I D-branes. The gauge group and transformation properties of the low energy effective field theory living in the corresponding unstable D-brane system are computed by extensive use of Clifford algebras. Some speculations about the existence of other versions of KK-groups, based on physical insights, are proposed. In the orbifold case, some known results concerning D-branes intersecting orbifolds are reproduced and generalized. Finally, the gauge theory of unstable systems in these orbifolds is recovered.