  We find the general structure for the two-gluon one-photon vertex in the
presence of a constant magnetic field. We show that, when accounting for the
symmetries satisfied by the strong and electromagnetic interactions under
parity, charge conjugation and gluon interchange, and for gluons and photons on
mass-shell, there exist only three possible tensor structures that span the
vertex. These correspond to external products of the polarization vectors for
each of the particles in the vertex. We also explicitly compute the one-loop
approximation to this vertex in the intermediate field strength regime, which
is the most appropriate one to describe possible effects of the presence of a
magnetic field to enhance photon emission during pre-equilibrium in peripheral
relativistic heavy-ion collisions. We show that the most favored direction for
the photon to propagate is in the plane transverse to the field, which is
consistent with a positive contribution to $\nu_2$ and may help to understand
the larger than expected elliptic flow coefficient measured in this kind of
reactions.
