  Linear equivalence is a criterion that compares submanifolds in the same
homology class. We show that, in the context of type II compactifications with
D-branes, this concept translates to the kinetic mixing between U(1) gauge
symmetries arising in the open and closed string sectors. We argue that in
generic D-brane models such mixing is experimentally detectable through the
existence of milli-charged particles. We compute these gauge kinetic functions
by classifying the 4d monopoles of a compactification and analyzing the Witten
effect on them, finding agreement with previous results and extending them to
more general setups. In particular, we compute the gauge kinetic functions
mixing bulk and magnetized D-brane U(1)'s and derive a generalization of linear
equivalence for these objects. Finally, we apply our findings to F-theory SU(5)
models with hypercharge flux breaking.
