In three-dimensional topological insulators (TI) the effective Maxwell action acquires a term that is absent in trivial insulators: the axion term. The form of the axion term implies that an electric field can induce a magnetic polarization, whereas a magnetic field can induce an electric polarization. It has been argued that the axion field in TIs can give rise to novel physical effects such as the formation of image magnetic monopoles. While this claim is rather controversial, I will point out other consequences of the axion electromagnetic response: vortex lines at a superconductor-TI interface induce a variant of the Witten effect so that each flux quantum attains a fractional electrical charge of e/4. This induces an ac Josephson effect in the absence of any external voltage. The fractionally charged quasiparticle also carries a fractional angular momentum.