SIMBAD references

The existence of a statistically significant excess of dwarf binaries with mass ratios from 0.95 to 1, as first noted by Lucy & Ricco (1979AJ.....84..401L), is confirmed by modern data. Excess of such binaries, called twins, is found only for periods shorter than 40 days, and in the 2-30 days period range they constitute 10-20% of the total binary population. Twins must have been formed by a special mechanism. It cannot be a mass transfer in contact pre-main-sequence binaries, fission is also unlikely. As shown by Bate (2000MNRAS.314...33B), accretion onto a close binary shifts mass ratio towards 1, so twins may originate from binaries that became close while still surrounded by massive envelopes. Many twins are members of higher-multiplicity systems and/or clusters, which probably explains an early formation of a close binary by stellar dynamics.