Circumstellar disks around stars older than 10 Myr are expected to be gas-poor .
There are , however , two examples of old ( 30–40 Myr ) debris-like disks containing a detectable amount of cold CO gas .
Here we present ALMA and Herschel Space Observatory observations of one of these disks , around HD 21997 , and study the distribution and origin of the dust and its connection to the gas .
Our ALMA continuum images at 886 µm clearly resolve a broad ring of emission within a diameter of \sim 4 \farcs 5 , adding HD 21997 to the dozen debris disks resolved at ( sub ) millimeter wavelengths .
Modeling the morphology of the ALMA image with a radiative transfer code suggests inner and outer radii of \sim 55 and \sim 150 AU , and a dust mass of 0.09 M _ { \oplus } .
Our data and modeling hints at an extended cold outskirt of the ring .
Comparison with the morphology of the CO gas in the disk reveals an inner dust-free hole where gas nevertheless can be detected .
Based on dust grain lifetimes , we propose that the dust content of this gaseous disk is of secondary origin produced by planetesimals .
Since the gas component is probably primordial , HD 21997 is one of the first known examples of a hybrid circumstellar disk , a so-far little studied late phase of circumstellar disk evolution .