Fig. 6.
Luminosity class criterion for O9–O9.7 stars. Left: Ratio of the equivalent widths of He II 4686 to He I 4713 as a function of luminosity class. Right: Ratio of the equivalent widths of Si IV 4089 to He I 4026 as a function of luminosity class. Gray circles and error bars indicate the average and standard deviation for each luminosity class. Dotted lines are the least square fits to the average ratios.
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