Do blue stragglers form by common envelope evolution

All proposals to explain the formation of blue stragglers advanced up to now seem to have inherent difficulties. One appealing possibility, the formation by coalescence of close binary stars, appears to be ruled out by the existence of blue straggers with more than twice the turn-off mass of their cluster. It is suggested that this contradiction does not exist, since the binary system on its way to coalescence passes through a phase in which a very extended common envelope with a size of several hundred solar radii forms. In this stage a third companion in the range of this envelope will be captured and finally merge to form a blue straggler that can have more than twice the turn-off mass. (Auth.)