HR diagram with model CBs, NCBs, and mergers at the age of 12 Gyr, together with the corresponding low-metal isochrone. Asterisks denote binaries with an orbital period shorter than 0.5 d, plus signs binaries with periods between 0.5 and 1 d, filled circles denote mergers formed between 10−11 Gyr, and open circles those formed between 11−12 Gyr. All mergers are evolved to the age of 12 Gyr. The ZAMS is marked with a dotted line and the adopted red boundary of the BS region with long dashes.