Top: fitting a representative summed spectrum of RC stars (black) using the ULySS program and ELODIE 3.2 spectral library. The model spectrum is shown in grey (green in the electronic version). Bottom: comparison of the spectrum of RC stars (black) with the one of HD 184406 from the MILES library (green in the electronic version). The differences between the object and model spectra are shown in the lower panels, where the dotted lines indicate zero differences.