V838 Cyg; O-C, amplitude & Fourier coeff modulations

The O-C diagram shows that the pulsation period of V838Cyg increases linearly over a long timescale. In a reanalysis of the high-precision Kepler data, we confirmed the modulation with a period of 59.45{+/-}0.07days found by Benko et al., and also found an additional weak modulation with a longer period (840{+/-}21 days).

After a series of analyses, we incline to the view that the mechanisms causing the two modulations are different: the former is more similar to the typical Blazhko effect, while the mechanism leading to the latter may be an extrinsic factor. We also collected and compared the modulation and physical parameters of other Blazhko RRLyrae stars from several works in the literature, and find that there is a potential negative correlation between the modulation amplitude (or upper limit of amplitude) and the metal abundance.