  We consider the interactions of strings on T-folds from the world-sheet point
of view which are exact in $\alpha'$. As a concrete example, we take a model
where the internal torus at the so(8) enhancement point is twisted by T-duality
(T-folded), and compute the scattering amplitudes of a class of massless
strings. The four-point amplitudes involving both twisted and untwisted strings
are obtained in a closed form in terms of the hypergeometric function. By their
factorization, the three-point coupling of the twisted and untwisted strings is
found to be suppressed by the chiral momenta along the internal torus, and
quantized in integer powers of 1/4. The asymptotic forms of the four-point
amplitudes in high-energy limits are also obtained. Our results rely only on
general properties of the asymmetric orbifold by the T-duality twist and of the
Lie algebra lattice from the symmetry enhancement, and thus may be extended
qualitatively to more general T-folds.
