Comparison of T_eff derived for the RGB stars using four different methods: three different colour-T_eff relations and our IRFM (see text for details). The T_eff from the colour-T_eff relations are shown as differences from the IRFM temperatures. Dashed lines are linear fits to each T_eff set, and the shaded area shows the average 2σ internal uncertainty of the IRFM scale (± 56 K). The 2σ uncertainties of the (b−y) and (B−V) relations are much larger than for our IRFM, being ± 140 and ± 192 K, respectively (Alonso et al. 1999). The (V−K) uncertainties (2σ = 46 K; González Hernández & Bonifacio 2009) are similar to the IRFM uncertainties.