B−V vs. U−B color–color diagrams. Color codes for points are the same as in Fig. A.1. The red line indicates the reddening vector for O3-O9 stars. The other lines are the calibration of color with spectral type of Allen (2000) for O5-M5 main sequence stars (green), G5-M5 giants (blue) and O3-M5 supergiants (pink).
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