  Noting the weakest modulation and relatively high metal abundance of the
ab-type RR Lyrae star V838 Cyg, we collected the photometric data of this star
from several sky surveys to carry out an in-depth analysis. The O-C diagram
shows that the pulsation period of V838 Cyg 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\pm0.07 days found by Benko et al., (2014),
and also found an additional weak modulation with a longer period (840\pm21
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 RR Lyrae 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. We
infer that the relatively high metal abundance will promote convection in the
outer stellar atmosphere, and then inhibit those factors (turbulence, shock
wave, etc.) that may cause Blazhko modulation. Future observations and research
work can be carried out with reference to this viewpoint. We also introduce the
moire effects that appear in the Kepler long-cadence light curves and their
possible interference in the previous analyses.
