@article{Dine1992CPAO,
title={CP and other gauge symmetries in string theory.},
author={Dine and Leigh and MacIntire},
journal={Physical review letters},
year={1992},
volume={69 14},
pages={
2030-2032
}
}

We argue that {ital CP} is a gauge symmetry in string theory. As a consequence, {ital CP} cannot be explicitly broken either perturbatively or nonperturbatively; there can be no nonperturbative {ital CP}-violating parameters. String theory is thus an example of a theory where all {theta} angles arise due to spontaneous {ital CP} violation, and are in principle calculable.

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