BPS orientifold planes from crosscap states in Calabi-Yau compactifications

We use the results of [1] on the simple current classification of open unoriented CFTs to construct half supersymmetry preserving crosscap states for rational Calabi-Yau compactifications. We show that the corresponding orientifold fixed planes obey the BPS-like relation M = e(iphi)Q. To prove this relation, it is essential that the worldsheet CFT property includes the degrees of freedom from the uncompactifted space-time component. The BPS-phase phi can be identified with the automorphism type of the crosscap states. To illustrate the method we compute crosscap states in Gepner models with each k(i) odd.