Predicted He ii λ1640 equivalent width as a function of metallicity for constant star formation (left panel) and very young (≤1−2 Myr) bursts (right panel). Note the different vertical scales on the two plots. Results for different IMFs are shown using the same colour codes as in Fig. 1 (cf. Table 1). Note that photoionization models predict generally fainter He ii λ1640 emission, hence lower equivalent widths, except for high ISM densities (see Sect. 4.3).