  We investigate the effects of exponentially correlated noise on birhythmic
van der Pol type oscillators. The analytical results are obtained applying the
quasi-harmonic assumption to the Langevin equation to derive an approximated
Fokker-Planck equation. This approach allows to analytically derive the
probability distributions as well as the activation energies associated to
switching between coexisting attractors. The stationary probability density
function of the van der Pol oscillator reveals the influence of the correlation
time on the dynamics. Stochastic bifurcations are discussed through a
qualitative change of the stationary probability distribution, which indicates
that noise intensity and correlation time can be treated as bifurcation
parameters. Comparing the analytical and numerical results, we find good
agreement both when the frequencies of the attractors are about equal or when
they are markedly different.
