Comparison between the NGC 1866 cluster data and selected isochrones in the F555W vs. F336W − F814W CMD. The data description is the same as in Fig. 1. The blue isochrone is selected taking the best values found from the common age and metallicity of the four-star combined PDF. The green and red isochrones are the two solutions of HV 12199 with low (CS-1) and high initial (CS-2) rotation rates, respectively. All the isochrones are plotted with the effect of gravity darkening. The inclination angles, i, are 0° (pole on) and 90° (edge on) for the most and less bright isochrones, respectively. The shaded area indicates the Cepheids instability strip.