A precise measurement of the cosmic microwave background radiation temperature from CN observations toward Zeta Persei

High-SNR high-resolution (R = 150,000-170,000) optical spectra of interstellar CN toward Zeta Per are presented. Fits to the data yield a value of 1.25 + or - 0.02 km/s for the R(O) linewidth parameter b along this line of sight. The column densities of the level populations are directly determined from the convolution of the instrument response function with a model which is then fitted to the data. Tex = 2.79 + or - 0.03 K is obtained along this line of sight.