CK Vulpeculae was observed in outburst in 1670–1672 (ref.1), but no counterpart was seen until 1982, when a bipolar nebula was found at its location1,2,3. Historically, CK Vul has been considered to be a nova (Nova Vul 1670), but its similarity to ‘red transients’, which are more luminous than classical novae and thought to be the results of stellar collisions4, has re-opened the question of CK Vul's status5,6. Red transients cool to resemble late M-type stars, surrounded by circumstellar material rich in molecules and dust7,8,9. No stellar source has been seen in CK Vul, though a radio continuum source was identified at the expansion centre of the nebula3. Here we report that CK Vul is surrounded by chemically rich molecular gas in the form of an outflow, as well as dust. The gas has peculiar isotopic ratios, revealing that CK Vul's composition was strongly enhanced by the nuclear ashes of hydrogen burning. The chemical composition cannot be reconciled with a nova or indeed any other known explosion. In addition, the mass of the surrounding gas is too large for a nova, though the conversion from observations of CO to a total mass is uncertain. We conclude that CK Vul is best explained as the remnant of a merger of two stars.