  Martin & Lubow (2017) showed through simulations that an initially tilted
disc around an eccentric binary can evolve to polar alignment in which the disc
lies perpendicular to the binary orbital plane. We apply linear theory to show
both analytically and numerically that a nearly polar aligned low mass
circumbinary disc evolves to polar alignment and determine the alignment
timescale. Significant disc evolution towards the polar state around moderately
eccentric binaries can occur for typical protostellar disc parameters in less
than a typical disc lifetime for binaries with orbital periods of order 100
years or less. Resonant torques are much less effective at truncating the inner
parts of circumbinary polar discs than the inner parts of coplanar discs. For
polar discs, they vanish for a binary eccentricity of unity. The results agree
with the simulations in showing that discs can evolve to a polar state.
Circumbinary planets may then form in such discs and reside on polar orbits.
