CH₃OCHO spectra of the 8₁₇−7₁₁ transitions at 100.48 GHz (A and E substates, bottom panel), of the 9₀₉−8₀₈ transitions at 100.68 GHz (A and E substates, middle panel), and of the 9₁₉−8₁₈ transitions at 100.08 GHz (A and E substates, top panel) in L1689B. The vertical dotted lines indicate the rest frequencies measured in the laboratory and listed in the JPL database (Pickett et al. 1998). The CH₃OCHO spectrum in the top panel also shows a HC₃N line. The negative spectral feature between both CH₃OCHO lines results from the frequency switching procedure and the folding of the ortho-CH₂CO line shown in Fig. 4.