  A measurement of the cross section of top quark pair production in
proton-proton collisions recorded with the ATLAS detector at the LHC at a
centre-of-mass energy of 7 TeV is reported. The data sample used corresponds to
an integrated luminosity of 2.05 fb^-1. Events with an isolated electron or
muon and a tau lepton decaying hadronically are used. In addition, a large
missing transverse momentum and two or more energetic jets are required. At
least one of the jets must be identified as originating from a b quark. The
measured cross section, sigma_ttbar = 186+/- 13 (stat) +/- 20 (syst) +/- 7
(lum.) pb is in good agreement with the Standard Model prediction.
