Parts of the 3 mm line survey of IRC +10216 showing the pairs of lines tentatively assigned to the N = 9−8 and N = 10−9 doublets of MgCCH. The rms noise levels per 1 MHz channel are 0.5 and 0.7 mK in the bottom and top panels, respectively. Thick gray lines correspond to the calculated line profiles under LTE, assuming that MgCCH extends between 5 and 20′′ with a homogeneous fractional abundance and rotational temperature of 20 K.