Oosterhoff dichotomy

One of the open problems concerning RRLs is the Oosterhoff dichotomy, namely the subdivision of GGCs into two distinct groups according to the average period < Pab > of their fundamental mode RRL variables (RRab, Oosterhoff 1939). In particular, in some GGCs containing RRLs, < Pab > is about 0.55 days while in others, it is close to 0.65 days, with a period gap between these two groups, named Oosterhoff I (OoI) and Oosterhoff II (OoII), respectively.