2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/5937The classical O--C curves are discussed in different cases in which various period changes involved. Among them, the analytic O--C curves with frequency, amplitude modulations and with double modes are closely inspected, respectively. As a special, the light-time effect is illustrated. The features of period change noise and period change to metallicity are added at the end.17 pages, 6 figuresAstrophysicstext