Enhanced axion-photon coupling in GUT with hidden photon
  We show that the axion coupling to photons can be enhanced in simple models
with a single Peccei-Quinn field, if the gauge coupling unification is realized
by a large kinetic mixing $\chi = {\cal O}(0.1)$ between hypercharge and
unbroken hidden U(1)$_H$. The key observation is that the U(1)$_H$ gauge
coupling should be rather strong to induce such large kinetic mixing, leading
to enhanced contributions of hidden matter fields to the electromagnetic
anomaly. We find that the axion-photon coupling is enhanced by about a factor
of 10-100 with respect to the GUT-axion models with $E/N = 8/3$.
