  CK Vulpeculae was observed in outburst in 1670-16721, but no counterpart was
seen until 1982, when a bipolar nebula was found at its location. Historically,
CK Vul has been considered to be a nova (Nova Vul 1670), but a similarity to
'red transients', which are more luminous than classical nova and thought to be
the result of stellar collisions, has re-opened the question of CK Vul's
status. Red transients cool to resemble late M-type stars, surrounded by
circumstellar material rich in molecules and dust. No stellar source has been
seen in CK Vul, though a radio continuum source was identified at the expansion
centre of the nebula. Here we report CK Vul is surrounded by chemically rich
molecular gas with peculiar isotopic ratios, as well as dust. 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 high 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.
