  Due to its interaction with the virtual electron-positron field in vacuum,
the photon exhibits a nonzero anomalous magnetic moment whenever it has a
nonzero transverse momentum component to an external constant magnetic field.
At low and high frequencies this anomalous magnetic moment behaves as
paramagnetic, and at energies near the first threshold of pair creation it has
a maximum value greater than twice the electron anomalous magnetic moment.
These results might be interesting in an astrophysical and cosmological
context.
