The HeI triplet to singlet line ratio Λ = F(λ5876)/F(λ6678) in symbiotic stars and novae is often much lower than the planetary nebulae value of ≈ 3.5, which is also the low density value Λ0 predicted by recombination theory.

Fig.1 shows that the dereddened HeI line ratios ^ are different for S type and D type symbiotic stars. The spectrophotometric data are mainly from Allen (1984, personal communication) and Blair et al.(1983). The S and D type classification divides the symbiotic stars according to the IR continuum into objects with a stellar continuum or dust emission. Practically all D types have low nebular densities Ne ≲ 107cm−3 and a rich spectrum of forbidden lines, while in S type objects these lines are suppressed by collisional deexcitation due to the high electron densities Ne ≳ 107cm−3. The observed HeI line ratio ^ in S type symbiotics, which is systematically too low when compared with the theoretical low density value Λ0 = 3.5, can therefore be attributed to the high nebular densities in these objects.