  Following a review of our present knowledge about blue subdwarf stars in
globular clusters, we present an overview of the results of searches for close
binaries among these stars, including results previously published in the
literature and reporting recent and preliminary results of new data. Previous
investigations revealed a lack of close systems in NGC 6752, which we confirm
with new, more extensive observations. Our estimate of the close binary
fraction in this cluster is only 4%. From a review of the relevant literature,
there are indications that a low close binary fraction among extreme horizontal
branch (EHB) stars is a common feature in globular star clusters. On the other
hand, the field EHB population shows evidence of a remarkably high binary
fraction. Such a difference among globular cluster and field populations,
although not yet explained in detail on the basis of theoretical models, must
necessarily be related to different formation histories for EHB stars in the
field and in clusters. In this framework, preliminary results indicate that
close systems could be relatively common in the peculiar globular cluster NGC
2808, although the sample of studied stars is still small. This would imply
that not all clusters share the same behavior, as far as EHB star formation is
concerned. We briefly explore possible explanations for these results.

Astrophysics