Determination of Cycle Length of Quasi-Periodic Signals. Application to
  Semiregular Variables
  We review methods for determination of "quasi-periods" (or "cycle length") of
signals of low coherence. Such type of variability was called "cyclic" for
semi-regular red variables, or "quasi-periodic oscillations" (QPO) for fast
variability in cataclysmic variables and related objects. Methods are
illustrated by application to AF Cygni.
