  The production of a W boson in association with a single charm quark is
studied using 4.6 fb^-1 of pp collision data at sqrt(s)=7 TeV collected with
the ATLAS detector at the Large Hadron Collider. In events in which a W boson
decays to an electron or muon, the charm quark is tagged either by its
semileptonic decay to a muon or by the presence of a charmed meson. The
integrated and differential cross sections as a function of the pseudorapidity
of the lepton from the W-boson decay are measured. Results are compared to the
predictions of next-to-leading-order QCD calculations obtained from various
parton distribution function parameterisations. The ratio of the
strange-to-down sea-quark distributions is determined to be 0.96 +0.26 -0.30 at
Q^2=1.9 GeV^2, which supports the hypothesis of an SU(3)-symmetric composition
of the light-quark sea. Additionally, the cross-section ratio sigma(W^+ +
bar{c})/sigma(W^- + c) is compared to the predictions obtained using parton
distribution function parameterisations with different assumptions about the
s-bar{s} quark asymmetry.
