Diagrams illustrating our two-step selection method to have a RC star population as homogeneous as possible among the Kepler (left), K2 (middle), and TESS-SCVZ fields (right) using E20 and APOGEE DR17. Top: T_eff vs. [Fe/H] diagrams. The red box indicates the first selection of the bulk of the RGB and RC stars to make sure that our sample is chemically similar. Bottom: Hertzsprung–Russell diagrams (T_eff vs. absolute magnitude from Gaia in the G-band) after the first selection was applied. We draw a region in the HRD to select RC stars (red rectangles).