Fig. 2.
Ratios EW(He I 4144)/EW(He II 4200) (left panel), EW(He I 4388)/EW(He II 4542) (middle panel), and EW(Si III 4552)/EW(He II 4542) (right panel) as a function of spectral type for O8–O9.7 stars (red triangles). Gray points and error bars indicate the average and dispersion. The gray dotted lines of the left and middle panels indicate a ratio of 1.0.
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