HIFI spectra of IRC +10216 showing the first three rotational lines of H³⁵Cl and H³⁷Cl (black histograms; spectral resolution is 1.5 MHz for the J = 1−0 and 3 MHz for the J = 2−1 and J = 3−2 lines). Line profiles resulting from the radiative transfer model are shown in magenta. For the J = 1−0 line, the calculated intensity has been divided among the three hyperfine components according to their line strengths.