Period Change Complexity in Classical Cepheids

Recent studies utilizing high-quality photometric data have revealed that many classical Cepheids exhibit low-amplitude variability phenomena, including subtle amplitude and phase modulations. These variations, often at the millimagnitude level, can coexist with short-term fluctuations in period measurements. Using the sample presented in Rathour et al. (2024, 2025), we investigate classical Cepheids showing additional low-amplitude variability, such as periodic modulation signatures and Cep0.68 variability, alongside detectable changes in their O–C diagrams indicative of erratic period change behaviour. In this talk, I will discuss how such low-amplitude variability impacts the interpretation of O–C analyses in classical Cepheids, illustrating how these phenomena can mimic or obscure genuine period changes. Understanding this connection is essential for refining period change measurements and interpreting Cepheid evolution accurately.