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

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 low-x regime. (C) 2008 Elsevier B.V. All rights reserved.