2013ApJ...774..153D

Twenty-one new optical light curves, including five curves obtained in 2009 and sixteen curves detected from the AAVSO International Database spanning from 1977 to 2011, demonstrate 16 new primary minimum light times in the high state. Furthermore, seven newly found low-state transient events from 2006 to 2009 were discovered, consisting of five Gaussian-shaped events and two events with an exponential form with decay timescales of <0.005 days; these timescales are one order of magnitude shorter than those of previous X-ray flare events. In the state transition, two special events were detected: a "disrupted event" with an amplitude of ∼2 mag and a duration of ∼72 minutes and continuing R-band twin events larger than all known R-band flares detected in M-type red dwarfs. All 45 available high-state data points spanning over 35 yr were used to construct an updated O - C diagram of AM Herculis, which clearly shows a significant sine-like variation with a period of 12-15 yr and an amplitude of 6-9 minutes. Using the inspected physical parameters of the donor star, the secular variation in the O - C diagram cannot be interpreted by any decided angular momentum loss mechanism, but can satisfy the condition τ_{dot}M2 ~= τ_KH ≫ τ_{dot}R2, which is required by numerical calculations of the secular evolution of cataclysmic variables. In order to explain the prominent periodic modulation, three plausible mechanisms–spot motion, the light travel-time effect, and magnetic active cycles–are discussed in detail.