  A systematic study on the long-term periodicities of known Galactic
cataclysmic variables (CVs) was conducted. Among 1580 known CVs, 344 sources
were matched and extracted from the Palomar Transient Factory (PTF) data
repository. The PTF light curves were combined with the Catalina Real-Time
Transient Survey (CRTS) light curves and analyzed. Ten targets were found to
exhibit long-term periodic variability, which is not frequently observed in the
CV systems. These long-term variations are possibly caused by various
mechanisms, such as the precession of the accretion disk, hierarchical triple
star system, magnetic field change of the companion star, and other possible
mechanisms. We discuss the possible mechanisms in this study. If the long-term
period is less than several tens of days, the disk precession period scenario
is favored. However, the hierarchical triple star system or the variations in
magnetic field strengths are most likely the predominant mechanisms for longer
periods.
