We report five photoluminescence lines N1 through N5 in silicon which emerge after sequential nitrogen and carbon implantation. Studied is in particular the 0.7456 eV (N1) electronic-vibronic spectrum. Single nitrogen and carbon atoms in the defect are identified by isotope shifts of the no-phonon transition and of a local mode satellite with vibration quantum energy ħω= 122.9 meV. Uniaxial stress or Zeeman measurements yield monoclinic I (C1h) or trigonal (C3v) symmetry, respectively, of the optical defect. Comparing the energy of the local mode and its isotope effects with recent literature data on the nitrogen 963 cm−1 IR vibrational absorption line we discuss a defect model involving a substitutional nitrogen atom modified by an interstitial carbon atom.