La, HoSeongNelson, Philip C2023-05-232023-05-231990-02-012017-04-19https://repository.upenn.edu/handle/20.500.14332/43284We show how to obtain loop-corrected effective field equations in tachyon-free heterotic string theories. The corrections are automatically gauge-invariant; they simultaneously secure Brst-decoupling and unambiguous loop amplitudes. We introduce a cutoff for two-dimensional field theory which is more general than the choice of a world-sheet metric, and using this we give an invariant description of factorization of fermionic amplitudes. As an example we work out the linearized corrections for the O(16) × O(16) heterotic string. They come from a finite corrected effective action.©. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Physical Sciences and MathematicsPhysicsArticle