  For self-sustained oscillators subject to noise the coherence, understood as
a constancy of the instantaneous oscillation frequency, is one of the primary
characteristics. The delayed feedback has been previously revealed to be an
efficient tool for controlling coherence of noise-driven self-sustained
oscillator. The effect of the delayed feedback control on coherence is stronger
for a longer delay time. Meanwhile, the instantaneous frequency of a noise-free
oscillator can exhibit multistability for long delay time. The impact of the
delay-feedback--induced multistability on the oscillation coherence, measured
by the phase diffusion constant, of a noisy oscillator is studied in this work
both numerically and analytically.
