  We study the r-modes and rotational ``hybrid'' modes of relativistic stars.
As in Newtonian gravity, the spectrum of low-frequency rotational modes is
highly sensitive to the stellar equation of state. If the star and its
perturbations obey the same one-parameter equation of state (as with isentropic
stars), there exist {\it no pure r-modes at all} - no modes whose limit, for a
star with zero angular velocity, is an axial-parity perturbation. Rotating
stars of this kind similarly have no pure g-modes, no modes whose spherical
limit is a perturbation with polar parity and vanishing perturbed pressure and
density. We compute the post-Newtonian corrections to the $l=m$ r-modes of
isentropic and non-isentropic uniform density stars.
