D-brane spectrum and K-theory constraints of D = 4, N = 1 orientifolds

We study the spectrum of stable BPS and non-BPS D-branes in Z_2 x Z_2 orientifolds for all choices of discrete torsion between the orbifold and orientifold generators. We compute the torsion K-theory charges in these D=4, N=1 orientifold models directly from worldsheet conformal field theory, and compare with the K-theory constraints obtained indirectly using D-brane probes. The K-theory torsion charges derived here provide non-trivial constraints on string model building. We also discuss regions of stability for non-BPS D-branes in these examples.