X-ray colour–colour diagram of the WR stars. WN stars are marked with star symbols and WC stars with circles, while open symbols correspond to optically/infrared-confirmed binary systems. Error bars are reported at the 90% confidence level. The grid shown with dotted grey lines represents the values of N_H and kT obtained from simulated absorbed thermal spectra. The grey arrows and the grey numbering indicate the direction in which the parameters of the two spectral components increase and their corresponding values, respectively.