MeerKAT Stokes-I spectrum (blue) of OH 18 cm lines at z = 0.89 towards PKS 1830−211. The velocity scale is defined with respect to z = 0.88582. The spectral rms in the region close to the 1667 MHz line is 2.9 mJy beam⁻¹ per 6.1 km s⁻¹ channel. The vertical dashed and dotted lines (black) represent the locations of the individual Gaussian components fitted to the absorption in main lines (M₁, M₂, and M₃; dotted red lines in (a) and (b)) and the excess absorption in the 1612 MHz satellite line (S₁ and S₂; dotted lines in (c); also see Table 1), respectively. The black solid line in panel c represents the absorption profile predicted under LTE due to M₁, M₂, and M₃. The difference between the red and black solid lines represents the maser contribution due to pumping, and is conjugate with the 1720 MHz line in (d): at V = −170 km s⁻¹, the 1720 MHz line is in emission and 1612 MHz in absorption, while at V = −80 km s⁻¹, the 1720 MHz is in absorption, and the 1612 MHz in emission. In all the panels, the total fit and the residuals, which are arbitrarily offset for clarity, are plotted as solid lines in red and magenta, respectively.