Example of GAM and Fourier fits for a handful M 51 Cepheids. The blue shaded region shows the 95th percentile range of the GAM fits. The plots show that the two models agree perfectly within these uncertainty limits. The grey shaded region shows the extension region for the GAM fitting, which was used to enhance the stability of the phase curve fits towards the edges of the region of interest (close to the phases −0.5 and 0.5).