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.