IT has long been suspected1 that OH–IR sources, highly evolved red giant stars that have built up massive, cool gaseous envelopes through heavy mass loss, are precursors to planetary nebulae. The two kinds of object share a similar galactic distribution, and their circumstellar envelopes have comparable masses and expansion velocities2,3. Recently, several hybrid objects have been found with the far infrared and OH maser emission characteristic of the OH–IR sources, but also with radio continuum emission from a central H II region4–6. There is strong circumstantial evidence that these objects, of which the prototype is Vy2–2 (refs 4,7), seem to be in a transitional state, but their precise evolutionary status remains unclear. Here we present radio-interferometer maps, obtained with the MERLIN network, of OH maser emission at 1,612 MHz from Vy27#150;2 and OH0.9 + 1.3, another of the hybrid sources. Our maps reveal shell structure in the masers, which we take to be conclusive evidence of an evolutionary link between OH–IR sources and planetary nebulae.