Contour plots of χ² of the fits at +20 and +52 days as function of the electron density and optical depth in the He Iλ 3889 line, τ(3889). The electron temperature was assumed to be T_e = 7 × 10³ K. When calculating χ², the He Iλ 7065, λ 6678, and λ 7291, line ratios provide the main constraints, while the λ 3889 and λ 4471 lines ratios are used as lower limits since they have strong P-Cygni absorption and weak net emission.