Q1 versus Q2 values, which are values independent of interstellar extinction. Plotted symbols are as in Fig. 1. The dashed line (Q2 = 11.25 × Q1 − 2.38 mag) serves as a first approximation to separate early and late-type stars. The dashed box with 0.1 < Q1 < 0.5 mag and 0.5 < Q2 < 1.5 mag indicates the selection of candidate RSGs (see text RSG2). WR stars populate the region enclosed by the two lines Q2 > 11.25 × Q1 − 2.38 mag and Q2 <  − 1.0 mag.