Evolution of the emission-line ratio He I/He II (left column) and T_* (right column) from evolutionary synthesis atmospheres from BPASS v.2.1 for different IMF and upper mass limits and metallicities as predicted from CLOUDY models and HCM-TEFF. The T_* values were derived with HCM-TEFF and vertical error bars show the uncertainty of these values as derived by the code. Panels in the upper row show predictions assuming the same Z both for the stars and the gas (i.e., Z_* = Z_g = 0.0022), while the panels of the lower row show predictions assuming nearly metal-free stars (Z_* = 10⁻⁵ and Z_g = 0.0022).