2024RNAAS...8...80R

V1391 Cassiopeiae is one of a growing number of novae to show emission from carbon monoxide but only the second to display molecular absorption as well (after V2676 Ophiuchi). In this Research Note, we present a 0.69-5.2 μm spectrum that includes both the CO fundamental and first overtone as well as a prominent, very low ionization spectrum dominated by the lines of C I. Relative to the CO first overtone, the CO fundamental is a factor of ten weaker than expected if the emission were optically thin. Modeling of the spectral structure of the first overtone indicates a CO temperature of 4000 K and ¹³C/¹²C of 0.5. CO emission coexisted with strong C I and O I emission so carbon monoxide formation did not fully incorporate either of the constituent elements.