Lines observed with the JCMT 15-m in comet 21P: HCN(4–3) at 354 505.472 MHz between 30 October and 2 November 1998, and HCN(3–2) at 265 886.434 MHz on 4.2 November 1998 UT. Negative ‘ghost’ lines appear at ± 13.7 km s⁻¹ (16.2 MHz) in the HCN(4–3) spectrum owing to the frequency switching technique. The position and relative intensity of the hyper-fine components are drawn below the spectra. The vertical scale is the main beam brightness temperature and the horizontal scale is the Doppler velocity in the comet rest frame.