At O9 the hydrogen lines show a more pronounced sensitivity to luminosity than at earlier (hotter) types. In addition, the ratio of Si IV 4089 (which increases in strength as we pass to higher luminosities) to H (which becomes narrower and weaker in the more luminous stars), the ratio of Si IV 4116 to the neighboring He I 4121 line, and the ratio of N III 4379 to He I 4387 can be used to judge the luminosity class at O9.