  We analyze the stabilization of unstable steady states by delayed feedback
control with a periodic time-varying delay in the regime of a high-frequency
modulation of the delay. The average effect of the delayed feedback term in the
control force is equivalent to a distributed delay in the interval of the
modulation, and the obtained distribution depends on the type of the
modulation. In our analysis we use a simple generic normal form of an unstable
focus, and investigate the effects of phase-dependent coupling and the
influence of the control loop latency on the controllability. In addition, we
have explored the influence of the modulation of the delays in multiple delay
feedback schemes consisting of two independent delay lines of Pyragas type. A
main advantage of the variable delay is the considerably larger domain of
stabilization in parameter space.
