Abstract :
We give an interpretation of the non-supersymmetric heterotic theories as the supersymmetric heterotic theories on Melvin background with Wilson lines. The coincidence of the partition functions is shown for most of the non-supersymmetric theories. We also discuss tachyonic instabilities in terms of nontrivial background fields. The behavior in the strong coupling region is investigated by using the duality relations of the supersymmetric theories.
