We study the CN radical using optical heterodyne magnetic rotation enhanced concentration modulation spec troscopy in the visible region. The radical has been produced in the ac glow discharge of acetonitrile with helium as the carrier gas. The (5, 0) band of the red system A2П - X2∑+ in the range 17450-17830cm-1 has been observed and rotationally analysed. We determine a set of precise molecular constants for the v = 5 vibrational level of CN in the A2ПI state.