Heavy-quark contributions to the ratio FL/F2 at low x

We study the heavy-quark contribution to the proton structure functions F2i(x,Q2) and FLi(x,Q2), with i=c,b, for small values of Bjorken's x variable at next-to-lading order and provide compact formulas for their ratios Ri=FLi/F2i that are useful to extract F2i(x,Q2) from measurements of the doubly differential cross section of inclusive deep-inelastic scattering at DESY HERA. Our approach naturally explains why Ri is approximately independent of x and the details of the parton distributions in the low-x regime.