  We describe results from a survey for J=3-2 12CO emission from visible stars
with an infrared excess. The line is clearly detected in 21 objects, with
molecular gas (>10^-3 Jupiter masses) common in targets with infrared excesses
>0.01 (>56% of objects). Such high excesses indicate the presence of a disc of
opening angle >12 degrees; within this, the optically thick disc prevents CO
photodissociation. Two or three stars with associated CO have an excess <0.01,
implying a disc opening angle <1 degree. Most line profiles are double-peaked
or relatively broad. Model fits, assuming a Keplerian disc, indicate outer
radii, R_out, of ~20-300 au. As many as 5 discs have outer radii smaller than
the Solar System (50 au), and a further 4 have gas at radii <20 au. R_out is
independent of the stellar spectral type (from K through to B9), but is
correlated with total dust mass. R_out appears to decrease with time: discs
around stars of age 3-7 Myr have a mean radius of ~210 au, whereas discs of age
7-20 Myr are a factor of 3 smaller. The only bona fide debris disc with
detected CO is HD9672; this has a double peaked line profile and is the most
compact gas disc observed, with a modelled radius 17 au). A fit to HD141569
suggests the gas lies in two rings of radii 90 and 250 au, similar to the
scattered light structure. In both AB Aur and HD163296 the sizes of the
molecular and dust scattering discs are also similar, suggesting that the gas
and small dust grains are co-located.
