  The increased sensitivity of millimeter-wave facilities now makes possible
the detection of low amounts of gas in debris disks. Some of the gas-rich
debris disks harbor peculiar properties, with possible pristine gas and
secondary generated dust. The origin of the gas in these hybrid disks is
strongly debated and the current sample is too sparse to understand this
phenomenon. More detections are necessary to increase the statistics on this
population. Lying at the final stages of evolution of proto-planetary disks and
at the beginning of the debris disk phase, these objects could provide new
insight into the processes involved in the making of planetary systems. We
carried out a deep survey of the 12CO(2-1) and 12CO(3-2) lines with the APEX
and IRAM radiotelescopes in young debris disks selected according to hybrid
disk properties. The survey is complemented with a bibliographic study of the
ratio between the emission of the gas and the continuum (S_CO/F_cont) in CTTS,
Herbig Ae, WTTS, hybrid, and debris disks. Our sub-mm survey comprises 25
stars, including 17 new targets, and we increase the sensitivity limit by a
factor 2 on eight sources compared to similar published studies. We report a
4sigma tentative detection of a double-peaked 12CO(2-1) line around HD23642; an
eclipsing binary located in the Pleiades. We also reveal a correlation between
the emission of the CO gas and the dust continuum from CTTS, Herbig Ae and few
debris disks. The observed trend of the gas to dust flux ratio suggests a
concurrent dissipation of the dust and gas components. Hybrid disks
systematically lie above this trend, suggesting that these systems may witness
a transient phase, when the dust has evolved more rapidly than the gas, with a
flux ratio S_CO/F_cont enhanced by a factor of between 10 and 100 compared to
standard (proto-)planetary disks.
