  We investigate the natural occurence of exponentially small couplings in
effective field theories deduced from higher dimensional models. We calculate
the coupling between twisted fields of the Z_3 Abelian orbifold
compactification of the heterotic string. Due to the propagation of massive
Kaluza-Klein modes between the fixed points of the orbifold, the massless
twisted fields located at these singular points become weakly coupled. The
resulting small couplings have an exponential dependence on the mass of the
intermediate states and the distance between the fixed points.
