Dereddened (H − K)₀ vs. K₀ diagram of the Galactic centre field using the data of Nishiyama et al. (2009) and the extinction map of Schultheis et al. (2009). In red, we indicate our selected M giants. The blue lines show the Padova isochrones with Z = 0.008, τ = 1 Gyr (solid line), and τ = 10 Gyr (dashed line), while the green line is at Z = 0.06 and τ = 10 Gyr. The arrow on the top right shows the extinction vector of A_K = 1 mag (Nishiyama et al. 2009).