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Recent investigations have revealed a surprising lack of close binaries amongextreme horizontal branch (EHB) stars in the globular cluster NGC6752, atvariance with the analogous sdB field stars. Another puzzling result concernsthe derived spectroscopic masses for some EHB stars. The present paper extendsour study of NGC6752 to M80 and NGC5986. Twenty-one horizontal branch stars(out of which 5 EHBs) in NGC5986 and 31 in M80 (11 EHBs) were observed duringfour consecutive nights. We measured radial velocity variations, temperatures,gravities, and helium abundances. By means of a statistical analysis, wedetected one EHB close binary candidate per cluster. In M80, the best estimateof the close binary EHB fraction is f=12%, and even the lowest estimate of thebinary fraction among field sdB stars can be ruled out within a 90% confidencelevel. Because of the small observed sample, no strong conclusions can be drawnon the close EHB binary fraction for NGC5986, although our best estimate israther low (f=25%). For the discrepancy in spectroscopic derived masses withtheoretical models observed in NGC6752, our analysis of M80 EHB stars shows asimilar trend. For the first time, we report a clear trend in surface heliumabundance with temperature. Our results show that the deficiency of closebinaries among EHB stars is now confirmed in two, and possibly three, globularclusters. This feature is therefore not a peculiarity of NGC6752. Our analysisalso proves that the strangely high spectroscopic masses among EHB stars arenow confirmed in at least a second cluster. Our results confirm that f could bea function of the age of the sdB star population, but we find that recentmodels have some problem reproducing all observations.

A lack of close binaries among hot horizontal branch stars in globular clusters - M80 and NGC5986