Late-type Wolf-Rayet (W-R) stars of the nitrogen sequence (WNL) can be sensibly divided into two groups: (1) WN6, WN7 and (2) the cooler WN8, WN9 stars. The latter generally have narrower emission lines and relatively strong He I P Cygni profiles. In either the LMC or the Galaxy, ~ 1/4 of all W-R stars are WNL; of these, ~ 1/4 are WN8,9. All massive stars that later become W-R stars, probably start as WNL. None of the nine monitored WN8,9 stars in the LMC and the Galaxy shows W-R + O binary-related radial velocity variations. This is in stark contrast with the 58% W-R + O binary frequency among the 26 monitored WN6,7 stars. This fraction is the same in each galaxy. Orbital masses of WN6,7 stars lie in the range ~ 30-60 M_sun_, with binary mass ratios M(WN6,7)/M(O) >~ 1. WN8,9 stars are much more dispersed in space than WN6,7 stars, which tend to be found in clusters. While WN8,9 stars have slightly fainter mean visual absolute magnitudes (M_v_ = -5.6 +/- 0.3) than WN6,7 stars (M_v_ = -6.1 +/- 0.2), possibly a result of crowding and duplicity of the WN6,7 sample, both groups show similar, relatively large dispersion in absolute magnitude [σ(M_v_) ~ 0.8-0.9 mag]. However, WN8,9 stars are considerably more variable than WN6,7 (or any other W-R) stars. The basic reason for the often strong differences between these two groups is unknown.
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A. F. J. Moffat (1989) studied this question.