Fundamental T_eff as a function of V−K colour index. Symbols and colours are as in Fig. 8. V magnitudes are mean values extracted from the GCPD (Mermilliod et al. 1997). K magnitudes were taken from the 2MASS catalogue (Cutri et al. 2003) if the entry had quality flag (Qflg) “A”. Otherwise they are mean values taken from the Catalog of Infrared Observations (Gezari et al. 2000), if available, and transformed to the 2MASS system (Carpenter 2001, Eq. (A1)). Stars with large error bars in V−K have K magnitudes from 2MASS with Qflg “D”. V−K for the Sun was taken from Casagrande et al. (2012). Grey line: empirical relation derived by Boyajian et al. (2013, their Eq. (2) and Table 8, row (V−K)^c.