Of CP and other Gauge Symmetries in String Theory
  We argue that \CP is a gauge symmetry in string theory. As a consequence, \CP cannot be explicitly broken either perturbatively or non-pertubatively; there can be no non-perturbative \CP-violating parameters. String theory is thus an example of a theory where all $\theta$ angles arise due to spontaneous \CP violation, and are in principle calculable. 
M. Dine, R.G. Leigh and D.A. MacIntire