Relation between the emission-line ratio He I/He II and the line ratios [O II]/[O III] for 68 objects of the control sample at z >  0.02 (at left), and [S II]/[S III] for another 84 objects at z <  0.02 (at right). Different symbols represent objects with (blue triangles) and without (orange circles) an observed WR bump. The lines represent predictions from models using BPASS 2.1 SEDs with an IMF with x = −1.35 and an upper mass limit of 300 M_⊙ considering different fractions of absorbed ionizing photons. Solid lines stand for models at 4 Myr with Z_* = Z_g = 0.0022, while dashed lines represent models at 6 Myr with Z_* = 10⁻⁵ and Z_g = 0.0022. Log U decreases towards the top right corner of each panel.