  Using a sample of dilepton top-quark pair (ttbar) candidate events, a study
is performed of the production of top-quark pairs together with heavy-flavor
(HF) quarks, ttbar+b+X or ttbar+c+X, collectively referred to as ttbar+HF. The
dataset used corresponds to an integrated luminosity of 4.7 fb-1 of
proton--proton collisions at a center-of-mass energy of 7 TeV recorded by the
ATLAS detector at the CERN Large Hadron Collider. The presence of additional HF
quarks in the ttbar sample is inferred by looking for events with at least
three b-tagged jets, where two are attributed to the b-quarks from the ttbar
decays and the third to additional HF production. The dominant background to
ttbar+HF in this sample is ttbar+jet events in which a light-flavor jet is
misidentified as a heavy-flavor jet. To determine the heavy- and light-flavor
content of the additional b-tagged jets, a fit to the vertex mass distribution
of b-tagged jets in the sample is performed. The result of the fit shows that
79 +/- 14 (stat.) +/- 22 (syst.) of the 105 selected extra b-tagged jets
originate from HF quarks, three standard deviations away from the hypothesis of
zero ttbar+HF production. The result for extra HF production is quoted as a
ratio (R_HF) of the cross section for ttbar+HF production to the cross section
for ttbar production with at least one additional jet. Both cross sections are
measured in a fiducial kinematic region within the ATLAS acceptance. R_HF is
measured to be [6.2 +/- 1.1 (stat.) +/- 1.8 (syst.)]% for jets with pT>25 GeV
and |eta|<2.5, in agreement with the expectations from Monte Carlo generators.
