Classical Cepheids are among the most important variable stars. As primary distance indicators, they are of key importance for stellar astrophysics, galactic astrophysics and cosmology. Stars pulsating in the fundamental mode, thanks to the characteristic light curve shape, and high intrinsic brightness are are among the most useful standard candles. They are believed to be regular pulsators Study of the-top quality photometry of the fundamental mode Cepheids from the Magellanic Clouds, collected by the Optical Gravitational Lensing Experiment, reveals that their pulsation is not so simple. In 51 fundamental mode Cepheids periodic modulation of the light curves was detected. Although the overall incidence rate is very low, about 1 per cent in each of the Magellanic Clouds, in the case of the SMC and pulsation periods between 12 and 16 d the incidence rate is nearly 40 per cent. On the other hand, in the LMC the highest incidence rate is 5 per cent for pulsation periods between 8 and 14 d, and the overall amplitude of the effect is smaller. It indicates that the phenomenon is metallicity dependent. Typical modulation periods are between 70 and 300 d. In nearly all stars the mean brightness is modulated, which, in principle, may influence the use of classical Cepheids for distance determination. Fortunately, the modulation of mean brightness does not exceed 0.01 mag in all but one star. Also, the effect averages out in typical observations spanning a long time base. Hence, in studies of sizeable samples of Cepheids with good phase coverage of the light curves, modulation has no practical effect on the distance determinations. It is clear however, that our understanding of the most important astrophysical candles is far from being complete.
The Figure shows a section of the observations of the modulated Cepheid, OGLE-SMC-CEP-1065 (dots) plotted along with the model (solid red line). Modulation of the light curve is clearly visible.
More information: R. Smolec, Unstable standard candles. Periodic light curve modulation in fundamental mode classical Cepheids, Monthly Notices of the Royal Astronomical Society (2017).