Criteria for separating galaxies into the upper and lower branches. On the left we have plotted the metallicity obtained with the Pilyugin & Grebel (2016) calibration against M_*. In orange we have plotted the OSSY data that meet the log N₂ < −0.6 criterion, defined by Pilyugin & Grebel (2016) as the branch separation. The blue contours in the background represent the OSSY objects in the upper branch, and the vertical dashed line marks a M_* value of log M_*(M_⊙) = 9.3, which seems to mark a limit for the objects in the lower branch. In the middle panel we show the relationship between the same metallicity and the parameter R₂₃. In the right panel we have plotted the parameter R₂₃ against M_*. The vertical line at log M_*(M_⊙) = 9.3 and the vertical line at R₂₃ = 0.85 mark the region where the lower branch objects tend to be.