  We study the heavy-quark contribution to the proton structure functions
F_2^i(x,Q^2) and F_L^i(x,Q^2), with i=c,b, for small values of Bjorken's x
variable at next-to-lading order and provide compact formulas for their ratios
R_i=F_L^i/F_2^i that are useful to extract F_2^i(x,Q^2) from measurements of
the doubly differential cross section of inclusive deep-inelastic scattering at
DESY HERA. Our approach naturally explains why R_i is approximately independent
of x and the details of the parton distributions in the small-x regime.
