  In part I, we use the post-Newtonian (pn) order of Liouville's equation to
study the normal modes of oscillation of a spherically symmetric relativistic
system. Perturbations that are neutral in Newtonian approximation develop into
a new sequence of normal modes.
  In part II, stability curve of r-modes of neutron stars are calculated by
considering vorticity-shear viscosity coupling.
  As an application, the loss of angular momentum through gravitational
radiation, driven by the excitation of r-modes, is considered in neutron stars
having rotation frequencies smaller than the associated critical frequency.
