  Embeddings of the standard model in type II string theory typically contain a
variety of U(1) gauge factors arising from D-branes in the bulk. In general,
there is no reason why only one of these - the one corresponding to weak
hypercharge - should be massless. Observations require that standard model
particles must be neutral (or have an extremely small charge) under additional
massless U(1)s, i.e. the latter have to belong to a so called hidden sector.
The exchange of heavy messengers, however, can lead to a kinetic mixing between
the hypercharge and the hidden-sector U(1)s, that is testable with near future
experiments. This provides a powerful probe of the hidden sectors and, as a
consequence, of the string theory realisation itself. In the present paper, we
show, using a variety of methods, how the kinetic mixing can be derived from
the underlying type II string compactification, involving supersymmetric and
nonsupersymmetric configurations of D-branes, both in large volumes and in
warped backgrounds with fluxes. We first demonstrate by explicit example that
kinetic mixing occurs in a completely supersymmetric set-up where we can use
conformal field theory techniques. We then develop a supergravity approach
which allows us to examine the phenomenon in more general backgrounds, where we
find that kinetic mixing is natural in the context of flux compactifications.
We discuss the phenomenological consequences for experiments at the low-energy
frontier, searching for signatures of light, sub-electronvolt or even massless
hidden-sector U(1) gauge bosons and minicharged particles.
