Spectra of the NO transitions detected in OH 231.8+4.2. The quantum numbers indicated in this plot for NO are Π and J (Table 3). Vertical thick segments indicate the rest frequency and relative strength of the individual hyperfine (hpf) components within each Π⁺ and Π⁻ blend (red and black, respectively). The LSR velocity scale is shown on top of the NO transitions that have been detected (the velocity scale refers to the strongest hpf component in each case). The LSR velocity of the emission feature associated to clump I4, at the base of the southern lobe, is indicated. The dotted line is the model spectrum (Sect. 3.2 and Table 4) assuming that the individual hpf components are centred at V_LSR = 28 km s^-1, and have an intrinsic width of FWHM = 25 km s^-1, similar to the dominant narrow component of the HNCO, HNCS, and HC₃N profiles (Figs. 3–6). These line parameters cannot explain the broad profiles and the apparent overall redshift of the NO blends, which are reproduced better by adopting larger FWHM and V_LSR (solid blue line: representative model with FWHM = 50 km s^-1 and V_LSR = 40 km s^-1).