Calculation of axion-photon-photon coupling in string theory

The axion search experiments invite a plausible estimation of the axion--photon--photon coupling constant $\overline{c}_{a\gamma\gamma}$ in string models with phenomenologically acceptable visible sectors. We present the calculation of $\overline{c}_{a\gamma\gamma}$ with an exact Peccei-Quinn symmetry. In the Huh-Kim-Kyae $\Z_{12-I}$ orbifold compactification, we obtain $\overline{c}_{a\gamma\gamma} =\frac{8}{3}$ even with a lot of singlets at the GUT scale, and the low-temperature axion search experiments will probe the QCD corrected coupling, ${c}_{a\gamma\gamma} \simeq \overline{c}_{a\gamma\gamma}-1.98\simeq 0.69$.