  For controlling periodic orbits with delayed feedback methods the periodicity
has to be known a priori. We propose a simple scheme, how to detect the period
of orbits from properties of the control signal, at least if a periodic but
nonvanishing signal is observed. We analytically derive a simple expression
relating the delay, the control amplitude, and the unknown period. Thus, the
latter can be computed from experimentally accessible quantities. Our findings
are confirmed by numerical simulations and electronic circuit experiments
