The axion search experiments invite a plausible estimation of the
axion--photon--photon coupling constant caγγ in
string models with phenomenologically acceptable visible sectors. We present
the calculation of caγγ with an exact Peccei-Quinn
symmetry. In the Huh-Kim-Kyae Z12−I orbifold compactification, we obtain
caγγ=38 even with a lot of singlets at the
GUT scale, and the low-temperature axion search experiments will probe the QCD
corrected coupling, caγγ≃caγγ−1.98≃0.69.Comment: LaTeX file of 7 page