Emissivities (j) of most important He I lines relative to the λ 5876 line. Upper panels: function of the optical depth in the λ 3889 line for two electron densities, n_e = 10⁶ cm⁻³ and n_e = 10⁷ cm⁻³, while bottom panels: function of the electron density for an optical depth in the He Iλ 3889 line, τ(3889) = 3.1 × 10³ (bottom left) and τ(3889) = 3.1 × 10⁴ (bottom right). The dashed colored lines give the corresponding observed line ratios for +20 days (dashed lines) and for +52 days (dotted), while the dashed vertical lines show the density and optical depth with the minimum chi square for these values of parameters. In all cases T_e = 7 × 10³ K.