  The presence of a plethora of light spin 0 and spin 1 fields is motivated in
a number of BSM scenarios, such as the axiverse. The study of the interactions
of such light bosonic fields with the Standard Model has focused mostly on
interactions involving only one such field, such as the axion ($\phi$) coupling
to photons, $\phi F \tilde F$, or the kinetic mixing between photon and the
dark photon, $ F F_D$. In this work, we continue the exploration of
interactions involving two light BSM fields and the standard model, focusing on
the mixed axion-photon-dark-photon interaction $\phi F \tilde F_D$. If either
the axion or dark photon are dark matter, we show that this interaction leads
to conversion of the CMB photons into a dark sector particle, leading to a
distortion in the CMB spectrum. We present the details of these unique
distortion signatures and the resulting constraints on the $\phi F \tilde F_D$
coupling. In particular, we find that for a wide range of masses, the
constraints from these effect are stronger than on the more widely studied
axion-photon coupling.
