  We discuss the effects of oblique internal magnetic fields on the spectrum of
type I superstrings compactified on tori. In particular we derive general
formulae for the magnetic shifts and multiplicities of open strings connecting
D9-branes with arbitrary magnetic fluxes. We discuss the flux induced potential
and offer an interpretation of the stabilization of R-R moduli associated to
deformations of the complex structure of T^6 in terms of non-derivative mixing
with NS-NS moduli. Finally we briefly comment on how to extract other low
energy couplings and generalize our results to toroidal orbifolds and other
configurations governed by rational conformal field theories on the worldsheet.
