Context. The molecular gas in protoplanetary disks can be traced with single-dish instruments in low J rotational lines of CO, in systems clear of coincident extended emission. Other rotational lines of CO also sample the gas-phase CO reservoir, albeit with different biases; CO(6-5) traces warmer molecular gas. A rarefied atomic gas could be traced in [C i](2-1), but no C i detections in disks exist. Aims: Our goals are to identify gas-rich systems that are bright in CO(6-5), for subsequent ALMA observations, and search for C i. Methods: We follow-up a CO(3-2) survey in protoplanetary disks with APEX/CHAMP maps in CO(6-5) 691.4 GHz and [C i](2-1) 809.3 GHz. Results: We obtain one compact CO(6-5) detection in HD 142 527, three upper limits, and extended CO(6-5) emission in HD 37389. Given the CO(2-1) flux, the CO(6-5) line in HD 142527 would imply a temperature of 19 K if the CO ladder was isothermal, for a common solid angle, which is close to CO freezeout. This low temperature, together with a modulated CO(6-5) line profile, can be explained by a 4? pointing offset. The C i observations yield upper limits on the mass of rarefied gas phase neutral carbon. Conclusions: The CO(6-5) detection in HD 142527 adds to the other two southern systems with known CO(6-5) signal (HD 100546 and TW Hya). The CO(6-5)/CO(3-2) flux ratio varies from 2 to 24 among seven sources with bona-fide CO(6-5) detections, and is uncorrelated with spectral type in this sample. The upper limits on CI emission constrain the fraction of carbon in low-density (. Given the CO(2-1) flux, the CO(6-5) line in HD 142527 would imply a temperature of 19 K if the CO ladder was isothermal, for a common solid angle, which is close to CO freezeout. This low temperature, together with a modulated CO(6-5) line profile, can be explained by a 4? pointing offset. The C i observations yield upper limits on the mass of rarefied gas phase neutral carbon. Conclusions: The CO(6-5) detection in HD 142527 adds to the other two southern systems with known CO(6-5) signal (HD 100546 and TW Hya). The CO(6-5)/CO(3-2) flux ratio varies from 2 to 24 among seven sources with bona-fide CO(6-5) detections, and is uncorrelated with spectral type in this sample. The upper limits on CI emission constrain the fraction of carbon in low-density (3 cm-3) atomic gas to ) atomic gas to -3 (3s). Based on observations carried out with the Atacama Pathfinder Experiment telescope (APEX). APEX is a collaboration between the Max-Planck Institut fr Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.
