Contact binaries are a kind of close binary system with very strong interaction. Long-term period variations imply the evolutionary state of contact binaries within the same time scale, which will determine their lifetime. It was believed that long-term period variations of contact binaries are dominated by their mass ratio q. Matter moves freely between the two components, which changes q and meanwhile changes the orbital periods (P).

Recently, LIU Liang's group at the Yunnan Observatories of Chinese Academy of Sciences found a high correlation between P and the fill-out factors (f) of contact binary systems which has been underestimated in previous studies. Based on this, they proposed an f-dominated mechanism of long-term period variations for contact binaries, which possibly reveals the other evolutionary track of contact binary systems. The study was published in MONTHLY NOTICES of the Royal Astronomical Society.

By the numerical method, LIU's group found that the observed long-term variations of P could also be caused by changes of fill-out factor (f). They believed that mass transfer and mass loss both occur via the common convective envelope (CCE), and different physical conditions of two components could lead to a different mass transfer rate. Thus, some matter will stagnate in CCE. If this part of the matter can be hold in CCE and can be released after a certain accumulation, a new evolutionary model called f-dominated model will be obtained.

In this new model, matter enters or leaves CCE periodically, transferring from one component to the other slowly. P and f should oscillate in whole process, meanwhile q continually decreases until the system merges. The f-dominated model is good to be applied to explain the presence of extremely short period, high mass ratio and deep contact binaries. This study revealed the complication of contact binary systems again, and indicated the importance of CCE in the evolution of contact binaries.