New spectroscopic investigations of the fourth-positive (A1Π → X1Σ+) system bands in the 13C16O molecule

►Thirteen bands of the first-positive system of 13C16O were analysed. ► Effective rovibronic molecular parameters and band origins were calculated. ► Real A1Π state rovibrational terms were calculated. ► Perturbations of v = 7, 9 and 12 levels were observed for the first time. ► Number of perturbating states and places of perturbation were verified and filled in.