2020MNRAS.492.4112Z

Secondary components of W UMa-type contact binaries (CBs) have many special properties, two of them are excess in radius and luminosity. In order to make these specialties clear, we propose radial density distribution to roughly detect the interior structure of the secondaries in CBs. By comparing the radial density distribution between secondary components of CBs and main-sequence stars, we find the radial density distribution of secondaries in A-subtype CBs are similar to main-sequence stars whose masses higher than 1.8 Msun, which suggests that these two kinds of stars may have some evolutionary relationship. While secondary components of W-subtype CBs are closely connected with stars whose masses lower than 1.8 Msun. Then, we investigate the mass-luminosity relation of secondaries in CBs, the big differences between two subtypes suggests that the overluminosity of secondaries in these two subtypes are caused by two different reasons. Overluminosity in A-subtype is because the secondary components are evolved from initial more massive stars, while in W-subtype is due to energy transfer.