  An increasing number of observations have shown that gaseous debris discs are
not an exception. However, until now we only knew of cases around A stars. Here
we present the first detection of 12CO (2-1) disc emission around an F star, HD
181327, obtained with ALMA observations at 1.3 mm. The continuum and CO
emission are resolved into an axisymmetric disc with ring-like morphology.
Using a Markov chain Monte Carlo method coupled with radiative transfer
calculations we study the dust and CO mass distribution. We find the dust is
distributed in a ring with a radius of 86.0 +- 0.4 AU and a radial width of
23.2 +- 1.0 AU. At this frequency the ring radius is smaller than in the
optical, revealing grain size segregation expected due to radiation pressure.
We also report on the detection of low level continuum emission beyond the main
ring out to ~200 AU. We model the CO emission in the non-LTE regime and we find
that the CO is co-located with the dust, with a total CO gas mass ranging
between 1.2x10^-6 Mearth and 2.9x10^-6 Mearth, depending on the gas kinetic
temperature and collisional partners densities. The CO densities and location
suggest a secondary origin, i.e. released from icy planetesimals in the ring.
We derive a CO+CO2 cometary composition that is consistent with Solar system
comets. Due to the low gas densities it is unlikely that the gas is shaping the
dust distribution.
