  Orientifold $p$-planes with $p\le4$ have fractional D$p$-charges, and
therefore appear inconsistent with Dirac quantization with respect to
D$(6-p)$-branes. We explain in detail how this issue is resolved by taking into
account the anomaly of the worldvolume fermions using the $\eta$ invariants. We
also point out relationships to the classification of interacting fermionic
symmetry protected topological phases. In an appendix, we point out that the
duality group of type IIB string theory is the pin+ version of the double cover
of $GL(2,\mathbb{Z})$.
