The binary second sequence in cluster colour-magnitude diagrams

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ABSTRA C T 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.