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

Organic molecules, ions, and rare isotopologues in the remnant of the stellar-merger candidate, CK Vulpeculae (Nova 1670)