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Recently we have observed a new emission spectrum of carbon monoxide (CO) in the near-infrared region, $8240cm-1$, which can he ascribed to the transition between $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma$ states. In order to identify the states, potential energy and transition dipole moment curves for the lowest four valence $(a3\Pi ,a\prime \mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma +,d3\Delta , and e\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma -)$ and the lowest three Rydberg $(b\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma +,j\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma +, and c3\Pi )$ triplet suites of CO were calculated theoretically by the highly correlated {ab initio} Multi-Configuration Self-Consistent Field and internally contracted Multi-Reference Configuration Interaction (MCSCF-MRCI) method with basis sets of double-zeta plus polarisation quality. Observed $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi -3\Sigma$ spectra can be assigned to none of the transitions between valence $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi$ and $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Sigma$ styles nor to those- between valence and Rydberg states. Only possibility remained is the transition between the low-lying c $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi$ and \ $b3\Sigma$ Rydberg states. The potential curves for these Rydberg states are isolated clearly enough to give simple emission spectrum, and the transition dipole moment between these states is very large, 8 Debye. These indicate that the newly observed spectrum should be attributed to the transition between c $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}\Pi$ and $b^{3}\Sigma^{+} $states.