  We analyse properties of the mass outflow from the Roche-lobe filling
component of a semi-detached binary system. We follow the approaches published
by Paczy\'nski \& Sienkiewicz and by Lubow \& Shu, which we compare with other
simplified approaches. We find that the density of the flow at $L_1$ is orders
of magnitude lower than the density on the same equipotential but away from
$L_1$. Furthermore, the effective cross section of the flow, after averaging
over its profile of the momentum density, is much lower than some published
estimates done without accounting for the averaging. Thus, the use of some
simplified formulae for the density and the flow cross section can lead to
overestimates of the accretion rate and of the mass contained in the $L_1$
regions by very large factors unless they are supported by simultaneous
integrations of the equations of stellar structure for the outer layers of the
donor.
