Muonic vacuum polarization correction to the bound-electron $g$-factor
  The muonic vacuum polarization contribution to the $g$-factor of the electron bound in a nuclear potential is investigated theoretically. The electric as well as the magnetic loop contributions are evaluated. We found these muonic effects to be observable in planned trapped-ion experiments with light and medium-heavy highly charged ions. The enhancement due to the strong Coulomb field boosts these contributions much above the corresponding terms in the free-electron $g$-factor. Due to their magnitude, muonic vacuum polarization terms are also significant in planned determinations of the fine-structure constant from the bound-electron $g$-factor. 
N. A. Belov, B. Sikora, R. Weis, V. A. Yerokhin, S. Sturm, K. Blaum,   C. H. Keitel, Z. Harman
physics.atom-ph