Hertzsprung-Russel diagram showing the pulsation cycle of the 17 modelled RRLs of our sample. Markers indicate the type of RV measurements used in the model (see legend of Fig. 14). Thick lines are the edges of the instability strip from models (Marconi et al. 2015) for a metallicity globally corresponding to RRLs. These predictions have been extended linearly to lower luminosities in semi-dotted lines.