1996A&A...305..541C

We investigate whether the stellar parameters and chemistries of Wolf-Rayet stars can be adequately determined from IR observations by focusing on two WN 6 stars which have been the subject of numerous optical studies. Our analysis is based on new UKIRT-CGS4 observations of the strong, broad-lined stars WR 134 (HD191765) and WR 136 (HD192163) in Cygnus. We find excellent agreement in our derived stellar parameters for both stars (T_*∼55kK, T_2/3∼29kK, logL/L_☉∼5.3, log{dot}(M)/M_☉/yr~-3.9, v_∞∼1900km/s) with previous optical tailored analyses (e.g. Hamann et al. 1994). The numerous unblended H and He emission lines in the IR allow us to determine very accurate H/He ratios for these two stars. We confirm that WR 136 contains substantial hydrogen (∼10% by mass) while less than 1% hydrogen is found for WR 134. We provide a spectral comparison of WN 3-9 stars in the I, H and K-bands and give theoretical intrinsic WN infrared colours of use for future IR studies. We note that the WN 9 star WR 105 (NS4) has a similar spectral morphology to the Galactic Centre helium emission stars (Allen et al. 1990; Krabbe et al. 1991). We also present synthetic 2.5-10µm spectra of WR 136 for comparison with planned ISO spectroscopic observations.