Variability of rms relative to the line strength σ as a function of mass of the Galactic H-free WN stars. The blue dots correspond to the small-scale variability in WN stars from St-Louis et al. (2009), Chené & St-Louis (2011), Michaux et al. (2014), while the pink dots correspond to the variability for the stars analysed in this work (see Appendix A). Empty black circles correspond to CIR-type variability (St-Louis et al. 2009; Chené & St-Louis 2011; St-Louis 2013). The stellar masses are derived by Hamann et al. (2006). The linear fit in blue dashed is computed considering only the blue and pink dots. The Spearman’s rank correlation coefficient is ≈0.7.