Context : The formation mechanism of hot horizontal branch ( HB ) stars is still one of the most uncertain points of stellar evolution theories .
In the past decade , models based on their binary origin have been very successful in reproducing the properties of field subdwarf-B stars , but the observations of their analogues in globular clusters has posed new problems .
In addition , the discovery of multiple populations offered an appealing alternative scenario for the formation of these stars .

Aims : We search for binaries of period \wp \leq 200 days among a sample of 83 blue horizontal branch stars ( T _ { \mathrm { eff } } =12 000-22 000 K ) in NGC 2808 , a cluster known to host three distinct stellar populations and a multimodal horizontal branch .
The final sample , after the rejection of stars with incomplete observations or poor quality data , consists of 64 targets .

Methods : The radial velocity of the targets was measured in fourteen epochs , spanning a temporal interval of \sim 75 days .
The significant variations were identified by means of a detailed error analysis and a statistical study .

Results : We detect no RV variable object among stars cooler than the photometric G1 gap at \sim 17 000 K , while two close ( \wp \leq 10 days ) and two intermediate-period ( \wp =10-50 days ) systems are found among hotter targets .
The close and intermediate-period binary fraction for stars cooler than the gap are f _ { \mathrm { c } } \leq 5 % and f _ { \mathrm { ip } } \leq 10 % , respectively , with 95 % confidence .
The most probable values among hotter stars are f _ { \mathrm { c } } \sim 20 % and f _ { \mathrm { ip } } \sim 30 % , but the 90 % - confidence level intervals are still large ( 6-42 % and 11-72 % , respectively ) .

Conclusions : The G1 gap appears as a discontinuity in the binary faction along the HB , with a higher incidence of binaries among hotter stars , but a constant increase in f with temperature rather than a discontinuity can not be excluded from our observations .
We also find that intermediate-period binaries , never investigated before among cluster HB stars , could play an important role among hotter stars , being more than \sim 15-20 % of the hottest stars of our sample .
Our results , compared with previous estimates for other clusters , indicate that f _ { \mathrm { c } } among hot HB stars is most probably higher for younger clusters , confirming the recently proposed age- f _ { \mathrm { c } } relation .
However , the large observed difference in binary fraction between clusters ( e.g .
NGC 2808 and NGC 6752 ) is still not reproduced by binary population synthesis models .