We study nonaxisymmetric perturbations of rotating relativistic stars .
modeled as perfect-fluid equilibria .
Instability to a mode with angular dependence \exp ( im \phi ) sets in when the frequency of the mode vanishes .
The locations of these zero-frequency modes along sequences of rotating stars are computed in the framework of general relativity .
We consider models of uniformly rotating stars with polytropic equations of state , finding that the relativistic models are unstable to nonaxisymmetric modes at significantly smaller values of rotation than in the Newtonian limit .
Most strikingly , the m=2 bar mode can become unstable even for soft polytropes of index N \leq 1.3 , while in Newtonian theory it becomes unstable only for stiff polytropes of index N \leq 0.808 .
If rapidly rotating neutron stars are formed by the accretion-induced collapse of white dwarfs , instability associated with these nonaxisymmetric , gravitational-wave driven modes may set an upper limit on neutron-star rotation .
Consideration is restricted to perturbations that correspond to polar perturbations of a spherical star .
A study of axial perturbations is in progress .