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

Enhanced axion-photon coupling in GUT with hidden photon