In an optical color-magnitude diagram sub-subgiants ( SSGs ) lie red of the main sequence and fainter than the base of the red giant branch in a region not easily populated by standard stellar-evolution pathways .
In this paper , we present multi-epoch radial velocities for five SSG candidates in the old and metal-rich open cluster NGC 6791 ( 8 Gyr , [ Fe/H ] = +0.30 ) .
From these data we are able to make three-dimensional kinematic membership determinations and confirm four SSG candidates to be likely cluster members .
We also identify three member SSGs as short-period binary systems and present their orbital solutions .
These are the first SSGs with known three-dimensional kinematic membership , binary status , and orbital parameters since the two SSGs in M67 studied by ( 22 ) .
We also remark on the other properties of these stars including photometric variability , H \alpha emission , and X-ray luminosity .
The membership confirmation of these SSGs in NGC 6791 strengthens the case that SSGs are a new class of nonstandard stellar evolution products , and that a physical mechanism must be found that explains the evolutionary paths of these stars .