Theoretical HR-diagram illustrating the photometric metallicity determination for Collinder 261 using the method by Pöhnl & Paunzen (2010). Logarithmic luminosity is plotted versus normalized logarithmic effective temperature, T_N =log T_eff – log T_eff(ZAMS), where T_eff (ZAMS) is the T_eff of a zero-age main-sequence with solar composition for a given luminosity (see Table 4 of Pöhnl & Paunzen 2010). Using the parameters by Paunzen & Netopil (2006), the metallicity was determined as Z = 0.021 ([Fe/H] ≈ 0.05 dex, isochrone shown as a thick line). In addition, the isochrone for Z = 0.012 ([Fe/H] ≈ − 0.2, thin line) is shown, representing the high-resolution metallicity value of Friel et al. (2003).