We report new potential energy curves for the X1Σ+ and A1Π states of the CO molecule by employing the realistic extended Hartree–Fock approximate correlation energy model for diatomics with inclusion of the united-atom limit (EHFACE2U). Special care has been taken to describe the A1Π potential maximum, since this affects the vibrational wave functions of the highest vibrational levels. The corresponding vibronic spectroscopy and Franck–Condon factors (v″=0, 1) have also been calculated using accurate ab initio transition dipole moments. Comparison has been made with recent theoretical and experimental data.