We present a detailed period analysis of the bright Cepheid-type variable star V1154 Cygni ( V =9.1 mag , P \approx 4.9 d ) based on almost 600 days of continuous observations by the Kepler space telescope .
The data reveal significant cycle-to-cycle fluctuations in the pulsation period , indicating that classical Cepheids may not be as accurate astrophysical clocks as commonly believed : regardless of the specific points used to determine the O - C values , the cycle lengths show a scatter of 0.015-0.02 days over the 120 cycles covered by the observations .
A very slight correlation between the individual Fourier parameters and the O - C values was found , suggesting that the O - C variations might be due to the instability of the light curve shape .
Random fluctuation tests revealed a linear trend up to a cycle difference 15 , but for long term , the period remains around the mean value .
We compare the measurements with simulated light curves that were constructed to mimic V1154 Cyg as a perfect pulsator modulated only by the light travel time effect caused by low-mass companions .
We show that the observed period jitter in V1154 Cyg represents a serious limitation in the search for binary companions .
While the Kepler data are accurate enough to allow the detection of planetary bodies in close orbits around a Cepheid , the astrophysical noise can easily hide the signal of the light-time effect .