Binarity as a possible explanation of the dual horizontal branch in NGC 6791

NGC 6791 is quite a famous old open cluster which harbors a dual red giant clump. A classical red clump, made of 45 stars, and a very blue clump (extended horizontal branch (EHB)), made of 12 stars. The origin of the second clump, composed of stars with virtually no envelope, is still disputed. Briefly, the lack of envelope is ascribed either to mass loss during the RGB phase, enhanced by the high cluster metallicity, or by binary evolution, and mass loss during the common envelope phase. It is crucial therefore to measure in a robust way the binary fraction of the cluster, especially for stars in the RGB phase. We will derive this percentage from a multi-epoch spectroscopic campaign conducted with FLAMES @ VLT. The plan of the thesis is to derive binary percentage through radial velocity distribution, and hence realize the possibility of binarity being the source for the formation of EHB stars.