ALMA Band 3 spectrum of SDP.81, extracted from the main Einstein arc. We derive the spectrum from dirty-image cubes with a 1-arcsec taper and a spectral resolution of 100 MHz. There is a clear positive excess at the position of the HCO⁺(4–3) line, as well as potential foreground CO(0–1) absorption (see Appendix A.1). Note: the line fluxes in Table 1 are extracted from the narrow-band images (Fig. 1).