Left-hand panels: folded (R = 50 000) model spectra showing He I 4922 Å, He I 5875 Å, and He I 6678 Å for fixed effective temperature T_eff = 28000 K, fixed surface gravity log(g) = 5.60, fixed total helium abundance log n(⁴He+³He) ∼ −2.00, but for four different combinations of ³He and ⁴He abundances. The corresponding isotopic helium abundances are log n(³He) = −4.00 and log n(⁴He) = −2.00 (black curve), log n(³He) = −2.30 and log n(⁴He) = −2.30 (red curve), and log n(³He) = −2.00 and log n(⁴He) = −4.00 (blue curve), respectively. In addition, a model spectrum for log n(³He) = −2.05 and log n(⁴He) = −3.05 is shown in magenta. Right-hand panels: same as the left-hand panels, but for a fixed total helium abundance of log n(⁴He+³He) ∼ −1.00.