  The L1544 prestellar core has been observed as part of the ASAI IRAM Large
Program at 3 mm. These observations led to the detection of many complex
molecules. In this Letter, we report the detection of two lines, at 85.5 GHz
(4,0,4-3,0,3) and 106.9 GHz (5,0,5-4,0,4), respectively, of the protonated
carbon dioxide ion, HOCO+. We also report the tentative detection of the line
at 100.4 GHz (5,0,5-4,0,4) of DOCO+. The non-LTE analysis of the detected lines
shows that the HOCO+ emission originates in the external layer where
non-thermal desorption of other species has previously been observed. Its
abundance is (5 +/- 2) e-11. Modelling of the chemistry involved in the
formation and destruction of HOCO+ provides a gaseous CO2 abundance of 2e-7
(with respect to H2) with an upper limit of 2e-6.
