  We present a systematic QCD analysis of the strange--charm and bottom--top
contributions to transverse and longitudinal structure functions in
charged--current deep inelastic scattering. Various ${\cal O}(\alpha_s^1)$
schemes are studied and compared. The dependence of their predictions on the
factorization scale $\mu^2$ is investigated. The theoretical uncertainties
resulting from the choice of the scheme and of $\mu^2$ are estimated.
