O-C diagrams for six of the seven Cepheids in our sample that are use to take into account the variation in the period in the calculation of the pulsation phase. For ℓ Car, ζ Gem, β Dor, η Aql, X Sgr and δ Cep, we consider the O-C values, the reference epoch and Heliocentric Julian date from Csörnyei et al. (2022). The O-C values (grey dots) are fitted by a parabola (dashed red line). The data used in this study (photometry and interferometry) are overplotted (see legend).