Obliquity Tides on Hot Jupiters
  Obliquity tides are a potentially important source of heat for extrasolar planets on close-in orbits. Although tidal dissipation will usually reduce the obliquity to zero, a nonzero obliquity can persist if the planet is in a Cassini state, a resonance between spin precession and orbital precession. Obliquity tides might be the cause of the anomalously large size of the transiting planet HD 209458b. 
Joshua N. Winn and Matthew J. Holman (Harvard-Smithsonian Center for   Astrophysics)