H-R diagram. The observationally derived positions of seven K–M giants in our sample are plotted by the filled diamonds with error bars. The theoretical evolutionary track from the main sequence to the asymptotic giant branch for a 2 M_⊙ star from Herwig (2005) is shown by the solid line. The evolutionary tracks from Bertelli et al. (2008) for 0.6, 1, and 1.5 M_⊙ stars with the solar metallicity (Z = 0.017) and a 1 M_⊙ star with a subsolar metallicity (Z = 0.004, appropriate for α Boo) are shown by the dots.