Stretched-period échelle diagram for the RGB star KIC 10866415 with a high inclination angle. Colours indicate the azimuthal order: the m = { − 1, 0, +1} rotational components are represented in green, light blue, and red, respectively. Observed modes are represented by dots and the symbol size varies as a function of the measured power spectral density. Rotational components with m = ±1 are identified in an automatic way through a correlation of the observed spectrum with a synthetic one constructed using Eqs. (5) and (6), represented here by crosses (Gehan et al. 2018). The location of the m = 0 rotational component is identified considering that the m = 0 ridge is median with respect to m = ±1 ridges. As there are six modes identified in the m = +1 ridge and eight modes in the m = −1 ridge, this gives a mean number of visible modes N_±1 = 7. We thus consider the first seven modes with the highest height-to-background ratio belonging to the m = 0 ridge.