Λ-doubling lines observed towards W Hya (used in the population diagram shown in Fig. 1) and IK Tau. The spectra are given in rest frequencies using the υ_LSR of W Hya (39.2 km s⁻¹) and IK Tau (33.8 km s⁻¹). The vertical dotted orange lines show the predicted line frequencies from the JPL catalogue. The arrows in the second panel from the bottom mark the lines identified as belonging to the OH molecule. The lines were extracted from the large region described in the text for W Hya and from within the central beam of the observations for IK Tau. Other lines can be seen in the displayed spectra, which we identified as follows: middle panel, the J_Ka, Kc = 42_10, 32 − 43_9, 35 SO₂ line (rest frequency 130.680 GHz); second panel from the bottom, a blend between J_Ka, Kc = 59_6, 54 − 58_7, 51 and 28_3, 25 − 27_4, 24 SO₂ lines (rest frequencies 333.360 and 333.364 GHz, and possibly also a blend with one line of ³⁴SO₂ and a vibrationally excited line of SO₂); and bottom panel, v, J_Ka, Kc = v₁, 7_4, 3 − 2v₂, 8_5, 4 H₂O line (tentative identification, rest frequency: 252.172 GHz).