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W UMa-type contact binaries belong to close binary systems whose componentsexactly overflow their Roche lobes and share a common convective envelope(CCE). In the last twenty years, the long-term variations of their orbitalperiods have been thought to depend on several mechanisms. Now, we suggest anew mechanism: CCE-dominated mechanism. The CCE-dominated mechanism is foundbased on our numerical result, especially at high mass ratios, that the orbitalperiods (P) of contact binaries change very much with their fill-out factors(f). Because f is taken as a measurement of the thickness of CCE, the physicalcause for the variation of P is a mass transfer between CCE and components.Further, an f-dominated simplification model for this mechanism is introduced.According to it, P may change in a long-term oscillation way with a similartime scale of the thermal modulation, meanwhile q is decreasing slowly till thetwo components merge. It could be also applied to explain the presence ofextremely short period, high mass ratio and deep contact binaries. Moreover,the CCE-dominated mechanism should always work due to mass transfer and massloss both occurring via CCE. Therefor, the effect of CCE on the variations oforbital periods may have been underestimated before.

A new mechanism of long-term period variations for W UMa-type contact binaries