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An increasing number of observations have shown that gaseous debris discs arenot an exception. However, until now we only knew of cases around A stars. Herewe present the first detection of 12CO (2-1) disc emission around an F star, HD181327, obtained with ALMA observations at 1.3 mm. The continuum and COemission are resolved into an axisymmetric disc with ring-like morphology.Using a Markov chain Monte Carlo method coupled with radiative transfercalculations we study the dust and CO mass distribution. We find the dust isdistributed in a ring with a radius of 86.0 +- 0.4 AU and a radial width of23.2 +- 1.0 AU. At this frequency the ring radius is smaller than in theoptical, revealing grain size segregation expected due to radiation pressure.We also report on the detection of low level continuum emission beyond the mainring out to ~200 AU. We model the CO emission in the non-LTE regime and we findthat the CO is co-located with the dust, with a total CO gas mass rangingbetween 1.2x10^-6 Mearth and 2.9x10^-6 Mearth, depending on the gas kinetictemperature and collisional partners densities. The CO densities and locationsuggest a secondary origin, i.e. released from icy planetesimals in the ring.We derive a CO+CO2 cometary composition that is consistent with Solar systemcomets. Due to the low gas densities it is unlikely that the gas is shaping thedust distribution.

Exocometary gas in the HD 181327 debris ring