The λ4050 bands, excited with the pure C13 isotope, have been photographed under high resolution. The lines of the strongest band show a 3: 1 intensity alternation. The B values of the C13 band are in the ratio 12: 13 to the B values of the corresponding C12 band. These isotope effects are interpreted as giving very good evidence that the λ4050 band is emitted by a triatomic carbon molecule C3.