Spectra of Lupus-1A and L483 around 86.65 GHz showing the quartet of emission lines assigned to HCCO and the doublet assigned to CH₂CHCH₃ in Lupus-1A. The rest frequency scale corresponds to a systemic velocity V_LSR = +5.0 km s^-1 (Lupus-1A) and +5.3 km s^-1 (L483). The rms is 0.0022 K (Lupus-1A) and 0.0034 K (L483) per 50 kHz channel. The bottom vertical marks indicate the position of the frequency switching negative artifacts, located at ±7.8 MHz from each line. Note that in the Lupus-1A spectrum, the intensity of the HCCO J = 9/2−7/2F = 4−3 and the A CH₂CHCH₃5_0,5−4_0,4 lines, whose frequencies differ by 7.7 MHz, is artificially reduced due to an overlap with frequency switching negative artifacts (thick bottom marks).