  We use three-dimensional hydrodynamical simulations to show that an initially
mildly misaligned circumbinary accretion disk around an eccentric binary can
evolve to an orientation that is perpendicular to the orbital plane of the
binary (polar alignment). As the disk evolves to the perpendicular state, it
undergoes nodal libration oscillations of the tilt angle and the longitude of
the ascending node. Dissipation within the disk causes the oscillations to
damp. The process operates above a critical initial misalignment angle that
depends upon the eccentricity of the binary and the mass of the disk. For
binary eccentricity of 0.5, the process operates typically for disk masses
smaller than a few percent of the binary mass and initial tilt angle of more
than 40 degrees. This evolution has important implications for planet formation
around eccentric binary star systems.
