Combination of the temperatures in one plot: from B − V corrected for interstellar extinction, from MK – >  T_eff, from obs1-obs26 (Sect. 2), and from some extra data (indicated by “X”(X_1 − 4), cf. text). The temperatures derived from the B − V and MK data combine reasonably well. The temperatures for obs1-obs2, obs4-obs6 also seem to follow the combined data, and the values for obs7-obs26 extend the data. The difference from obs2 is relatively large. This forms step 5.