We show how the second sequence seen lying above the main sequence in cluster colour-magnitude diagrams results from binaries with a large range of mass ratios and not just from those with equal masses. We conclude that the presence of a densely populated second sequence, with only sparse filling in between it and the single star main sequence, does not necessarily imply that binary mass ratios are close to unity.
Monthly Notices of the Royal Astronomical Society,
Vol. 300, no. 4 (November 1998), p. 977-980