  We analyze the Callan-Gross ratio $R(x,Q^2)=F_L/F_T$ in heavy-quark
leptoproduction as a probe of the charm content of the proton. To estimate the
charm-initiated contributions, we use the ACOT($\chi$) variable-flavor-number
scheme. Our analysis shows that charm densities of the recent CTEQ sets of
parton distributions have sizeable impact on the Callan-Gross ratio in a wide
region of $x$ and $Q^2$. In particular, the ACOT($\chi$) predictions for the
quantity $R(x,Q^2)$ are about half as large as the corresponding expectations
of the photon-gluon fusion mechanism for $x\sim 10^{-2}-10^{-1}$ and $Q^2\gg
m^2$. This is because the structure functions $F_T (x,Q^2)$ and $F_L (x,Q^2)$
have different dependences on the mass logarithms of the type
$\alpha_{s}\ln(Q^{2}/m^{2})$. On the other hand, our recent studies indicate
that, contrary to the production cross sections, the Callan-Gross ratio is
sufficiently stable under radiative corrections to the photon-gluon fusion
component for $x\gtrsim 10^{-4}$. We conclude that the quantity $R(x,Q^2)$ in
heavy-quark leptoproduction is perturbatively stable but sensitive to
resummation of the mass logarithms. For this reason, in contrast to the
structure functions, the ratio $R(x,Q^2)=F_L/F_T$ could be good probe of the
charm density in the proton.
