Perturbations of orthogonal polynomials with periodic recursion coefficients

The results of Denisov-Rakhmanov, Szeg-Shohat-Nevai, and Killip-Simon are extended from asymptotically constant orthogonal polynomials on the real line (OPRL) and unit circle (OPUC) to asymptotically periodic OPRL and OPUC. The key tool is a characterization of the isospectral torus that is well adapted to the study of perturbations.