# Unstable standard candles. Periodic light curve modulation in fundamental mode classical Cepheids

## TL;DR

This study reports the discovery of periodic modulation in fundamental mode classical Cepheids, revealing a metallicity-dependent phenomenon that could influence distance measurements but generally has negligible effects.

## Contribution

It is the first large-scale identification of pulsation modulation in classical Cepheids, highlighting its dependence on metallicity and potential impact on astrophysical distance calculations.

## Key findings

- Modulation occurs in about 1% of Cepheids, up to 40% in certain Magellanic Cloud populations.
- Modulation periods range from 70 to 300 days, with minimal impact on brightness measurements.
- The phenomenon's origin may be linked to a resonance between pulsation modes.

## Abstract

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.