  We consider a hydrogenic system with a nucleus of finite mass. The coupling
of the radiation field to the center of mass motion gives rise to an effective
scalar type coupling. This induced scalar photon interaction emerges as a
correction in competition with the usual multipole interactions. This effect is
particularly important in positronium where the electric quadrupole interaction
is totally suppressed. We illustrate this effect with the two-photon decay of
metastable hydrogenic systems.
