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The asymptotic-giant-branch star R Sculptoris is surrounded by a detached shell of dust and gas^,. The shell originates from a thermal pulse during which the star underwent a brief period of increased mass loss^,. It has hitherto been impossible to constrain observationally the timescales and mass-loss properties during and after a thermal pulse—parameters that determine the lifetime of the asymptotic giant branch and the amount of elements returned by the star. Here we report observations of CO emission from the circumstellar envelope and shell around R Sculptoris with an angular resolution of 1.3″. What was previously thought to be only a thin, spherical shell with a clumpy structure is revealed to also contain a spiral structure. Spiral structures associated with circumstellar envelopes have been previously seen, leading to the conclusion that the systems must be binaries^,,,. Combining the observational data with …