Temporal variation of the pulsation period of the twin-peak Cepheids (solid thick black line), together with its bootstrapped pointwise CI (thin black lines). Heavy orange line: catalog period, orange band: 95% nonparametric bootstrap CI around this period based on the bootstrap procedureof Sect. A.2. The dotted black vertical lines indicate years after HJD − 2 450 000. The gray background highlights time intervals where the deviation from the stable reference model was found significant (cf. Sect. A.3). The times of the observations are indicated with a rugplot at the bottom of the panels.