Stratification in WR envelopes as seen in HeII lines.

Using literature data on lines widths we show that the HeII emission lines originate in different regions of the Wolf-Rayet (WR) envelope. It is concluded that because of variations of lines widths within one series of an ion the Ionization Potential vs. Full Width at Half Maximum (FWHM) velocity diagrams can not be used for quantitative analysis of WR stars. Another physical quantity such as line optical depth should be used. For simplicity we use n_u (principal quantum number of upper level) or EP - excitation energy of the upper level in (u-1) transition. The correlation is strongest with n_u, indicating the importance of optical depth effects on the line formation region. The velocity run in the envelopes of 5 Wolf-Rayet stars HD 4004, 191765, 192163, 193928, 50896 is discussed on the basis of the He II (n-3), (n-4) and (n-5) series emissions lines widths on the n_u or EP vs. FWHM velocity graphs.