  CK Vul is a star whose outburst was observed in 1670-72. A stellar-merger
event was proposed to explain its ancient eruption. Aims: We aim to investigate
the composition of the molecular gas recently discovered in the remnant of CK
Vul. Methods: We observed millimeter and submillimeter-wave spectra of CK Vul
using the IRAM 30m and APEX telescopes. Radiative-transfer modeling of the
observed molecular features was performed to yield isotopic ratios for various
elements. Results: The spectra of CK Vul reveal a very rich molecular
environment of low excitation ($T_{\rm ex} \lesssim$12 K). Atomic carbon and
twenty seven different molecules, including two ions, were identified. They
range from simple diatomic to complex polyatomic species of up to 7 atoms
large. The chemical composition of the molecular gas is indicative of carbon
and nitrogen-driven chemistry but oxides are also present. Additionally, the
abundance of F may be enhanced. The spectra are rich in isotopologues that are
very rare in most known sources. All stable isotopes of C, N, O, Si, and S are
observed and their isotopic ratios are derived. Conclusions: The composition of
the remnant's molecular gas is most peculiar and gives rise to a very unique
millimeter and submillimeter spectrum. The observation of ions and complex
molecules suggests the presence of a photoionizing source but its nature (a
central star or shocks) remains unknown. The elemental and isotopic composition
of the gas cannot be easily reconciled with standard nucleosynthesis but
processing in hot CNO cycles and partial He burning can explain most of the
chemical peculiarities. The isotopic ratios of CK Vul are remarkably close to
those of presolar nova grains but the link of Nova 1670 to objects responsible
for these grains is unclear.
