The significance of binaries with nearly identical components

The distribution of the mass ratios of double-lined spectroscopic binaries is investigated, and a narrow peak at q of about 0.97 is found after observational errors have been corrected for. A variety of observational and theoretical arguments are then used to demonstrate that this peak cannot be explained as being due to the effects of either evolution or selection. The peak is therefore interpreted as implying that many close binaries (P less than about 25 days) with intermediate and small total masses are formed by a mechanism that, in its ideal form, would create binaries with identical components. Recent calculations suggest that this mechanism be identified with binary formation by fragmentation during the final dynamical-collapse phase of a rotating protostar's pre-main-sequence evolution.