Selected ²⁸SiC₂ lines observed in IRC +10216 with the IRAM 30 m telescope with a spectral resolution of 195 kHz. Abscissa is frequency in MHz and the intensity scale is in antenna temperature. The fitted line profile (see Sect. 3) is shown in red. Quantum numbers for the rotational transitions shown in this figure are indicated at the top-right corner of each panel. Measured and fitted rest frequencies are given in Table A.2.