Mass–radius and mass–T_eff diagrams of the NGC 6866 giants. Upper panel: mass–radius diagram. Black filled circles mark the measured masses and radii of the NGC 6866 giants according to Eqs. (1) and (2). Red symbols mark the same values and those corresponding to adding, in turn, 1σ to Δν (leftmost point), ν_max, and T_eff (rightmost point). Blue symbols mark the mean masses of Eqs. (2)–(5) and radii from the Stefan–Boltzmann equation. The red and blue plus signs mark the mean masses and radii of the red and blue symbols, respectively, not counting the most evolved star. Details of the isochrones are given in the legend. Lower panel: mass–T_eff diagram. Isochrones are the same as in the upper panel. Blue symbols mark the mean masses of Eqs. (2)–(5) and T_eff values from APOGEE DR17.