Continuum-subtracted HIFI spectra of H₂O, CO, OH, CH⁺, OH⁺ obtained towards the central position of Ser SMM1 (ν_source = 8.5 km s^-1). The red vertical line indicates ν_source, while the blue dashed line shows the position of the offset component. The blended OH triplet is centred on the strongest hyperfine component as indicated by the three vertical black lines situated directly beneath the OH spectrum. The same is the case for the OH⁺ spectrum, with the location of the hyperfine components and their relative strengths indicated by the black lines above the spectrum. The spectra have been shifted vertically for clarity and in some cases scaled by a factor, indicated on the figure. CH⁺ and OH⁺ are fitted by a single Gaussian, OH by two, and CO and H₂O by three; the offset components are shown in magenta.