Upper panel: Classification of the star along the HR diagram: O-type (black), WNh (blue), WN (orange), and WC and WO (red). Lower panel: Surface abundances as a function of the effective temperature, T_eff. Line styles are for different elements: hydrogen (dotted line), ¹⁴N (solid line), ¹²C (dashed line). Different colors are for different initial masses: 20 (blue), 30 (orange), 40 (red), and 50 M_⊙ (green). Green triangles (circles) represent hydrogen (nitrogen) abundances for the five hottest WRs observed (Hainich et al. 2015). The gray horizontal band covers the ∼5 − 40% range; if the hydrogen abundance is inside this region, the star is considered a WNh.