Spectroscopic binaries with components of similar mass
  The assertion that there is an intrinsic excess of binaries with mass ratios q \simeq 1 - the twin hypothesis - is investigated. A strong version of this hypothesis (H_s), due to Lucy & Ricco (1979) and Tokovinin (2000), refers to a narrow peak in the distribution function psi(q) for q \ga 0.95. A weak version (H_w), due to Halbwachs et al. (2003), refers to a broad peak for q \ga 0.8. Current data on SB2's is analysed and H_s is found to be statistically significant for a sample restricted to orbits of high precision. But claims that H_s is significant for binaries with special characteristics are not confirmed since the sample sizes are well below the minimum required for a reliable test. With regard to H_w, additional observational evidence is not presented, but evidence to the contrary in the form of Hogeveen's (1992b) model of biased sampling with psi \propto q^{-2} is criticized. Specifically, his success in thus fitting catalogued data depends on implausible assumptions about the research methodologies of binary-star spectroscopists. 
L.B.Lucy (Astrophysics Group, Imperial College)