Measurement of the Microwave Background Temperature from CN Absorption

Observations of the interstellar lines of CN near 3875A toward ζ Oph provide precise measurements of the Microwave Background Radiation (MBR) temperature and determine the intrinsic linewidth necessary for the saturation correction. We report TMBR =2.74±0.05 K at λ=2.64 mm and TMBR =2.75 (+0.24, -0.29) K at λ=1.32 mm.MBR