Abelian gauge symmetries other than the massless hypercharge of the Standard Model appear extensively in string theoretical approaches to particle physics. Generically, the corresponding gauge bosons acquire Stuckelberg masses by absorbing axionic scalar fields, rendering the symmetries effectively global at low energies.
It is well known that instanton effects break these global symmetries. I will review a simple mechanism by which abelian discrete remnants are preserved even at the non-perturbative level, as well as its generalization to the non-abelian case. Potential phenomenological applications will also be discussed.
