Examples of cluster supergiants (black) which appear earlier than O9–B0 Ia template spectra (red; Hanson et al. 2005) by virtue of ubiquitous and pronounced He II 2.189 μm absorption and C IV 2.079 μm emission. Even when the strength of He II 2.189 μm absorption is comparable to the O9 Ia template (e.g. LHO44), the combination of C IV 2.079 μm emission and/or much weaker He I 2.112 μm absorption clearly marks them out as of earlier spectral type. We therefore adopt a classification of O7–8 Ia, noting that the broad Brγ emission in LHO54 leads to a classification of O7–8 Ia^{( + )} (Sects. 3.2 and 3.3). HST/NICMOS F205W magnitudes are given in parentheses (Table A.1).