tokens: HO (under laboratory conditions), which is consistent with the astronomical observation of solid CO in water-rich environments. Neill et al. ( 2014 ) performed a sensitive line survey of Sgr B2(N), and based on the HOCO species, they estimated a CO/CO ratio in the colder external envelope of between 0.010.1, implying a CO gas-phase abundance of 10–10, consistent with the finding of Minh et al. ( 1988 , 1991 ). Their lower limit to the CO gas-phase abundance of 10 relative to H indicates that the CO abundance in the gas is clearly enhanced, while most of the water is still frozen out in ices in the Sgr B2 envelope. Table 1 Properties of the observed HOCO and DOCO lines ( K). Notes. The upper limits have been computed using the rms around the line. The errors in the fluxes include the calibration and statistical uncertainties from the Gaussian fits of the lines. As part of the IRAM-30 m Large Program ASAI 1 (Lefloch et al., in prep.), we carried out a highly sensitive, unbiased spectral survey of the molecular emission of the L1544 prestellar core with high spectral resolution. This source is a prototypical starless core in the Taurus molecular cloud complex ( pc) on the verge of the gravitational collapse (Caselli et al. 2002 , and references within). It is characterised by a central high density ( cm) and a low temperature (7 K). We reported the detection of many oxygen bearing complex organic molecules produced through the release in the gas phase of methanol and ethene through non-thermal desorption processes (Vastel et al. 2014 , hereafter Paper I). The sensitivity of these IRAM observations also led to the detection of the hyperfine structure of CHCN (Vastel et al. 2015a ). In the present study we report on the detection of the HOCO ion in the L1544 prestellar core, as well as a tentative detection of its deuterated form, DOCO. 2. Observations and results The observations were performed at the IRAM-30 m towards L1544 () using the broad-band receiver EMIR at 3 mm, connected to an FTS spectrometer in its 50 kHz resolution mode. The beam of the observations is 29 and 23 at 85 and 106 GHz, respectively. Line intensities are expressed in units of main-beam brightness temperature (see Paper I for more details). The frequencies and other spectroscopic parameters of the HOCO transitions have been retrieved from the JPL database 2 from laboratory measurements by Bogey et al. ( 1988 ). Two lines of HOCO (the 4–3 and 5–4 at 85.531 and 106.913 GHz, respectively) lie in the frequency range covered by the ASAI survey for upper level energies lower than 30 K and were detected. Their spectra are shown in Fig. 1 . Table 1 reports the spectroscopic parameters as well as the properties of the two detected lines, obtained by Gaussian fitting. For the deuterated form of HOCO, DOCO, we used the JPL database with the spectroscopic parameters from Bogey et al. ( 1986 , 1988 ). Three transitions lie in the frequency range for upper level energies lower than 30 K.
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