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An era has started in which gas and dust can be observed independently inprotoplanetary disks, thanks to the recent surveys with ALMA. The firstnear-complete high-resolution disk survey in both dust and gas in a singlestar-forming region has been carried out in Lupus, finding surprisingly lowgas/dust ratios. The goal of this work is to fully exploit CO isotopologuesobservations in Lupus, comparing them with physical-chemical model results, inorder to obtain gas masses for a large number of disks. We have employedphysical-chemical models to analyze continuum and CO isotopologues observationsof Lupus disks, including isotope-selective processes and freeze-out. Employingalso the ALMA 13CO-only detections, disk gas masses have been calculated for atotal of 34 sources, expanding the sample of 10 disks studied by Ansdell et al.(2016), where also C18O was detected. We confirm that overall gas-masses arevery low, often smaller than 1 $M_{\rm J}$, if volatile carbon is not depleted.Accordingly, global gas/dust ratios predominantly between 1 and 10. LowCO-based gas masses and gas/dust ratios may indicate rapid loss of gas, oralternatively chemical evolution, e.g. via sequestering of carbon from CO tomore complex molecules, or carbon locked up in larger bodies. Current ALMAobservations cannot distinguish between these two hypotheses. We have simulatedboth scenarios, but chemical model results do not allow us to rule out one ofthe two. Assuming that all Lupus disks have evolved mainly due to viscousprocesses over the past few Myr, the observed correlation between the currentmass accretion rate and dust mass found by Manara et al. (2016) implies aconstant gas-to-dust ratio, which is close to 100 based on the observed $M_{\rmdisk}/\dot{M}_{\rm acc}$ ratio. This in turn points to a scenario in whichcarbon depletion is responsible for the low CO isotopologue line luminosities.

Lupus disks with faint CO isotopologues: low gas/dust or large carbon depletion?