The Kepler Cepheid V1154 Cyg revisited: light curve modulation and detection of granulation

We present a detailed analysis of the bright Cepheid-type variable star V1154 Cygni using 4 yr of continuous observations by the Kepler space telescope. We detected 28 frequencies using the standard Fourier transform method. We identified modulation of the main pulsation frequency and its harmonics with a period of ∼159 d. This modulation is also present in the Fourier parameters of the light curve and the O - C diagram. We detected another modulation with a period of about 1160 d. The star also shows significant power in the low-frequency region that we identified as granulation noise. The effective time-scale of the granulation agrees with the extrapolated scalings of red giant stars. Non-detection of solar-like oscillations indicates that the pulsation inhibits other oscillations. We obtained new radial velocity observations that are in a perfect agreement with previous years data, suggesting that there is no high-mass star companion of V1154 Cygni. Finally, we discuss the possible origin of the detected frequency modulations.