Cosmic Background Radiation at 1.32 mm

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