Couplings between QCD axion and photon from string compactification
  The QCD axion couplings of various invisible axion models are presented. In
particular, the exact global symmetry U(1)$_{\rm PQ}$ in the superpotential is
possible for the anomalous U(1) from string compactification, broken only by
the gauge anomalies at one loop level, and is shown to have the resultant
invisible axion coupling to photon, $c_{a\gamma\gamma}\ge
\frac83-c_{a\gamma\gamma}^{\rm ch\,br}$ where $c_{a\gamma\gamma}^{\rm
ch\,br}\simeq 2$. However, this bound is not applicable in approximate
U(1)$_{\rm PQ}$ models with sufficiently suppressed U(1)$_{\rm PQ}$-breaking
superpotential terms. We also present a simple method to obtain
$c_{a\gamma\gamma}^0$ which is the value obtained above the electroweak scale.
