a Sternberg Astronomical Institute, Moscow, Russia
b Astronomical Institute, Tashkent, Uzbekistan
c Tashkent University, Tashkent, Uzbekistan
d S. A. Astronomical Observatory, Cape Town, South Africa
Abstract:
The secular evolution of the pulsation period of δ Cephei, η Aquilae, and ζ Geminorum during a cumulative total of 577 years suggests wave-like departures about the smooth time trends.