Donor mass vs. orbital period for CBSs, for the case of a typical GC chemical composition (Z = 0.001) in the region where redbacks are located together with the corresponding observational data. Symbols denote minimum masses for radio pulsar companions. Circles and squares represent redbacks and accreting MSPs, whereas the triangle denotes the case of a system that switches between these states, all from the Patruno’ catalogue¹. The thick solid (dotted) line corresponds to the mass radius relationship expected for Population II (I) composition objects given by Tauris & Savonije (1999).