Detected trans-HCOOH rotational lines towards the cold cloud Barnard 1-b. The ordinate refers to the intensity scale in main beam temperature units and the abscissa to the Doppler velocity. Line frequencies (in GHz) are indicated at the top of each panel together with the rotational quantum numbers. The red curve shows an excitation model that reproduces the rotational population diagram. The bottom panel shows the stacked spectra for cis-HCOOH.