Mass transfer in cataclysmic variables ( CVs ) is usually considered to be caused by angular momentum loss ( AML ) driven by magnetic braking and gravitational radiation ( GR ) above the period gap , and solely by GR below the period gap .
The best-fit revised model of CV evolution recently by Knigge et al .
( 17 ) , however , indicates that AML rate below the period gap is 2.47 ( \pm 0.22 ) times the GR rate , suggesting the existence of some other AML mechanisms .
We consider several kinds of consequential AML mechanisms often invoked in the literature : isotropic wind from the accreting white dwarfs , outflows from the Langrangian points , and the formation of a circumbinary disk .
We found that neither isotropic wind from the white dwarf nor outflow from the L _ { 1 } point can explain the extra AML rate , while ouflow from the L _ { 2 } point or a circumbinary disk can effectively extract the angular momentum provided that \sim ( 15 - 45 ) \% of the transferred mass is lost from the binary .
A more promising mechanism is a circumbinary disk exerting gravitational torque on the binary .
In this case the mass loss fraction can be as low as \lesssim 10 ^ { -3 } .