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We present measurements of the semileptonic decays B- --> D0 tau- nubar_tau,B- --> D*0 tau- nubar_tau, B0bar --> D+ tau- nubar_tau, and B0bar --> D*+ tau-nubar_tau, which are sensitive to non--Standard Model amplitudes in certainscenarios. The data sample consists of 232x10^6 Upsilon(4S) --> BBbar decayscollected with the BaBar detector at the PEP-II e+e- collider. We select eventswith a D or D* meson and a light lepton (ell=e or mu) recoiling against a fullyreconstructed B meson. We perform a fit to the joint distribution of leptonmomentum and missing mass squared to distinguish signal B --> D(*) tau-nubar_tau (tau- --> ell- nubar_ell nu_tau) events from the backgrounds,predominantly B --> D(*) ell- nubar_ell. We measure the branching-fractionratios R(D) == BR(B --> D tau- nubar_tau) / BR(B --> D ell- nubar_ell) andR(D*) == BR(B --> D* tau- nubar_tau) / BR(B --> D* ell- nubar_ell) and, from acombined fit to B- and B0bar channels, obtain the results R(D)=(41.6 +/- 11.7+/- 5.2)% and R(D*)=(29.7 +/- 5.6 +/- 1.8)%, where the uncertainties arestatistical and systematic. Normalizing to measured B- --> D(*)0 ell- nubar_ellbranching fractions, we obtain BR(B --> D tau- nubar_tau)=(0.86 +/- 0.24 +/-0.11 +/- 0.06)% and BR(B --> D* tau- nubar_tau)=(1.62 +/- 0.31 +/- 0.10 +/-0.05)%, where the additional third uncertainty is from the normalization mode.We also present, for the first time, distributions of the lepton momentum,p*_ell, and the squared momentum transfer, q^2.

Measurement of the Semileptonic Decays B --> D tau- nubar_tau and B --> D* tau- nubar_tau