Abstract
Cataclysmic variables (CVs) are close binary systems where the late-type star (the secondary) overfills its Roche lobe and transfers matter to a white dwarf (the primary) via an accretion disc. In this poster we report the first results of long-term project to study cyclical period variations in CVs. The observations were done from 2008 to 2013 at Observatório do Pico dos Dias (OPD/LNA, Brazil). Times series of high speed CCD photometry were obtained using the 0.6 m and 1.6 m telescopes at OPD. We measured new white-dwarf mid-eclipse timings and combined them with published measurements to construct updated observed-minus-calculated (O-C) diagrams. The UU Aqr O-C diagram covers 24 years of observations and presents a 26 yr modulation with semi-amplitude of 47 s. The V2051 Oph data cover 35 years of observations and the new timings show significant deviations from the published linear plus sinusoidal ephemeris (22 yr modulation with a semi-amplitude of 17 s), indicating that the variation is not strictly periodic. We discuss the observed modulations in context of the two current explanations for the phenomenon: magnetic activity in the secondary star and the presence of a third body in the system.
