Unstable standard candles. Periodic light curve modulation in
  fundamental mode classical Cepheids
  We report the discovery of periodic modulation of pulsation in 51 fundamental
mode classical Cepheids of the Magellanic Clouds observed by the Optical
Gravitational Lensing Experiment. 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 16d 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 14d, and the overall amplitude of the
effect is smaller. It indicates that the phenomenon is metallicity dependent.
Typical modulation periods are between 70 and 300d. 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. Consequently,
the effect of modulation on the determination of the distance moduli is
negligible. The relative modulation amplitude of the fundamental mode is also
low and, with one exception, it does not exceed 6 per cent. The origin of the
modulation is unknown. We draw a hypothesis that the modulation is caused by
the 2:1 resonance between the fundamental mode and the second overtone that
shapes the famous Hertzsprung bump progression.
