We present a novel technique for stabilizing unstable periodic orbits
of chaotic systems. It uses a continuous feedback loop in the form of
the difference between an actual and a delayed output signal of the sy
stem with a variable delay time. This time is chosen to be equal to th
e interval between the last and the kappa th previous maximum of the o
utput signal and is changed at every kappa th maximum. During the proc
edure, the delay time asymptotically tends to the period of the period
-kappa unstable periodic orbits and the control signal vanishes. The m
ethod is illustrated with the help of the Rossler ordinary differentia
l equations and the Mackey-Glass delay differential equations.