  Spin-transfer torque in asymmetric spin valves can destabilize both parallel
and antiparallel configurations and can lead to precessional modes also in the
absence of an external magnetic field. We find a bistable precessional regime
in such systems and show that thermal fluctuations can excite transitions
(telegraph noise) between the corresponding oscillatory regimes that are well
separated by irreversible paths at low temperatures. Because of the thermally
induced transitions, the frequency of the resulting current-driven oscillations
is different from that obtained at very low temperatures. We also show that the
power spectrum in the bistable region is dominated by the out-of-plane
oscillatory mode.
