ALMA narrow-band images of SDP.81. Upper left: rest-frame 847-μm continuum at native resolution; the main Einstein arc and the counter-image are clearly visible. The point-source emission in the centre of the image is from the previously identified AGN in the lensing galaxy at z = 0.299. Upper right and lower panels: narrow-band images at the systemic frequencies of the HCN(4–3), HCO⁺(4–3), and HNC(4–3) lines. The images are collapsed over 430 km s⁻¹ bandwidth. Contours start at ±2σ, with a 1σ increment. The significantly larger beam in the HNC image is caused by the lack of long-baseline observations at this frequency. The HCN(4–3) and HNC(4–3) lines are not detected, but HCO⁺(4–3) is clearly detected and resolved.