Non-renormalization of Yukawa couplings on orbifolds

We discuss the non-renormalization of Yukawa couplings in the orbifold compactification of the heterotic string, at the one-loop level. Special attention is paid to on-shell infrared divergences. The non-vanishing part of the one-loop quark-quark-quark scattering amplitude is identified with the one-loop correction of the external legs.