Upper panel: comparison of our spectroscopically derived with the Q-based computed with Eqs. (8)−(10). The dashed line indicates the 1:1 relation. The error bar shows a typical conservative uncertainty from our spectroscopic determination, and the uncertainty in , accounting for typical uncertainties in the colours only. Lower panel: percent difference of the two T_eff-values for the individual objects. These quantify the systematic differences from application of our Q–T_eff calibrations. The dotted lines indicate the 1σ-scatter range.