  We report sub-arcsec CO observations of the disks around MWC 480, LkCa 15 and
DM Tau, and simultaneous measurements of HCO+ J=1-0. We derive the disk
properties by fitting a standard disk model, with all parameters power laws of
the distance to the star. Biases are explained and discussed. We find evidence
for vertical temperature gradient in the disks of MWC 480 and DM Tau, as in AB
Aur, but not in LkCa 15. The disks temperature increase with stellar effective
temperature. Most of the CO gas is at temperatures smaller than 17 K, the
condensation temperature on grains. The scale height of the CO distribution
appears larger (by 50%) than the hydrostatic scale height. The more UV luminous
stars have more CO, but there is no simple dependency of CO abundance and
isotopologue ratio with stellar type. The 13CO/HCO+ ratio is around 600. The
temperature behaviour is consistent with expectations, but published chemical
models have difficulty reproducing the observed CO quantities. Vertical mixing
and photo-dissociation at the disk outer edge seem important chemical agents.
The CO data suggest a more complex surface density distribution than assumed in
models.
