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Models based on their binary origin have been very successful in reproducingthe properties of field subdwarf-B stars, but the observations of theiranalogues in globular clusters has posed new problems, while the discovery ofmultiple populations offered an appealing alternative scenario for theformation of these stars. We search for binaries of period P<200 days among asample of blue horizontal branch stars (Teff=12000-22000 K) in NGC2808, acluster known to host three distinct stellar populations and a multimodalhorizontal branch. The final sample consists of 64 targets. The radial velocityof the targets was measured in fourteen epochs, spanning a temporal interval ofabout 75 days. We detect no RV variable object among stars cooler than thephotometric G1 gap at 17000 K, while two close (P<10 days) and twointermediate-period (P=10-50 days) systems are found among hotter targets. Theclose and intermediate-period binary fraction for stars cooler than the gap arefc<5% and fip<10%, respectively, with 95% confidence. The most probable valuesamong hotter stars are fc~20% and fip~30%, but the 90%- confidence levelintervals are large (6-42% and 11-72%, respectively). The G1 gap appears as adiscontinuity in the binary faction, with a higher incidence of binaries amonghotter stars, but a constant increase in f with temperature rather than adiscontinuity cannot be excluded from our observations. We find thatintermediate-period binaries, never investigated before among cluster HB stars,could play an important role, being more than ~15-20% of the hottest stars ofour sample. Our results indicate that fc among hot HB stars is most probablyhigher for younger clusters, confirming the recently proposed age-fc relation.However, the large observed difference in binary fraction between clusters(e.g. NGC2808 and NGC6752) is still not reproduced by binary populationsynthesis models.

A lack of close binaries among hot horizontal branch stars in globular clusters. II. NGC\,2808