  We study type I compactification on a 4-torus, with a non-trivial discrete
background RR 4-form field. By using string dualities and recent insights for
gauge theories on tori, we find that a non-trivial background for the RR 4-form
is correlated with Spin(32)/Z_2 bundles that are described by a ``non-trivial
quadruple'' of holonomies. We also briefly discuss other discrete moduli for
the type I string, and variants of orientifold planes.
