Cosmic Background Radiation at 1.32 mm

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The position of the $R(0)$, $R(1)$, and $R(2)$ lines of the 3874-\AA{} band of interstellar CN in the spectrum of the star $\ensuremath{\zeta}$ Ophiuchi were scanned with a Fabry-Perot interferometer. The Doppler width (full width at half-maximum) of the $R(0)$ line is 28 m\AA{}. At the expected position of the $R(2)$ line an absorption feature was found with an equivalent width of 0.077 \ifmmode\pm\else\textpm\fi{} 0.055 m\AA{} which implies a temperature of ${2.9}_{\ensuremath{-}2.1}^{+3.4}$ K for the cosmic background radiation at 1.32 mm. A conservative interpretation of the data is simply to give an upper limit ($2\ensuremath{\sigma}$) of 3.8 K to this temperature.