  CO is an important component of a protoplanetary disc as it is one of the
most abundant gas phase species. Furthermore, observations of the CO
transitions can be used as a diagnostic of the gas, tracing conditions in both
the inner and outer disc. We present Herschel/PACS spectroscopy of a sample of
22 Herbig Ae/Be (HAEBEs) and 8 T Tauri stars (TTS), covering the pure
rotational CO transitions from J = 14-13 up to J = 49-48. CO is detected in
only 5 HAEBEs, AB Aur, HD 36112, HD 97048, HD 100546 and IRS 48, and in 4 TTS,
AS 205, S CrA, RU Lup, and DG Tau. The highest transition seen is J = 36-35,
with Eup of 3669 K, detected in HD 100546 and DG Tau. We construct rotational
diagrams for the discs with at least 3 CO detections to derive Trot, and find
average temperatures of 270 K for the HAEBEs and 485 K for the TTS. HD 100546
requires an extra temperature component with Trot ~ 900-1000 K, suggesting a
range of temperatures in its disc atmosphere, consistent with thermo-chemical
disc models. In HAEBEs, the objects with CO detections all have flared discs in
which the gas and dust are thermally decoupled. We use a small model grid to
analyse our observations and find that an increased amount of flaring means
higher line flux, as it increases the mass in warm gas. CO is not detected in
our flat discs as the emission is below the detection limit. We find that HAEBE
sources with CO detections have high Luv and strong PAH emission, again
connected to the heating of the gas. In TTS, the objects with CO detections are
all sources with evidence for a disc wind or outflow. For both groups of
objects, sources with CO detections generally have high UV luminosity (either
stellar in HAEBEs or due to accretion in TTS), but this is not a sufficient
condition for the detection of the far-IR CO lines.
