  Type II string compactifications to 4d generically contain massless
Ramond-Ramond U(1) gauge symmetries. However there is no massless matter
charged under these U(1)'s, which makes a priori difficult to measure any
physical consequences of their existence. There is however a window of
opportunity if these RR U(1)'s mix with the hypercharge $U(1)_Y$ (hence with
the photon). In this paper we study in detail different avenues by which
$U(1)_{RR}$ bosons may mix with D-brane U(1)'s. We concentrate on Type IIA
orientifolds and their M-theory lift, and provide geometric criteria for the
existence of such mixing, which may occur either via standard kinetic mixing or
via the mass terms induced by St\"uckelberg couplings. The latter case is
particularly interesting, and appears whenever D-branes wrap torsional
$p$-cycles in the compactification manifold. We also show that in the presence
of torsional cycles discrete gauge symmetries and Aharanov-Bohm strings and
particles appear in the 4d effective action, and that type IIA St\"uckelberg
couplings can be understood in terms of torsional (co)homology in M-theory. We
provide examples of Type IIA Calabi-Yau orientifolds in which the required
torsional cycles exist and kinetic mixing induced by mass mixing is present. We
discuss some phenomenological consequences of our findings. In particular, we
find that mass mixing may induce corrections relevant for hypercharge gauge
coupling unification in F-theory SU(5) GUT's.
