Spectroscopy of 47 WN stars and 36 WC stars in the Galaxy in the near-infrared is presented. In the WN types, the strongest lines are due to He II transitions and N IV at the 7115 A. In WC types, the strongest lines are due to C III, particularly at 9711 A; C II transitions, the strongest at 7236 A, and C IV lines, particularly at 7726 A, are also found. It is a relatively easy procedure to determine the W-R class from this wavelength region; similarly, the distinction between WNE ('early') and WNL ('late'), and between WCE ('early') and WCL ('late') may also be made, although numerical W-R subtypes cannot easily be determined from this near-infrared region. A comparison of line ratios of the various ions found in W-R stars suggests that it is only the optical N IV 4057 A and C III 5696 A transitions which do not behave in a 'well-understood' manner; curiously enough, it is just these lines that play an important role in the traditional numerical subtype definitions.
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Conti et al. (1990) studied this question.