  In this paper, we generalize Szego's theorem for orthogonal polynomials on
the real line to infinite gap sets of Parreau-Widom type. This notion includes
Cantor sets of positive measure. The Szego condition involves the equilibrium
measure which is shown to be absolutely continuous. Our approach builds on a
canonical factorization of the M-function and the covering space formalism of
Sodin-Yuditskii.
