We investigate a new constraint on new-physics interpretations of the anomalies observed in B→D^{(*)}τν decays making use of the lifetime of the B_{c}^{-} meson. A constraint is obtained by demanding that the rate for B_{c}^{-}→τ^{-}ν[over ¯] does not exceed the fraction of the total width that is allowed by the calculation of the lifetime in the standard model. This leads to a very strong bound on new-physics scenarios involving scalar operators since they lift the slight, but not negligible, chiral suppression of the B_{c}^{-}→τ^{-}ν[over ¯] amplitude in the standard model. The new constraint renders a scalar interpretation of the enhancement measured in R_{D^{*}} implausible, including explanations implementing extra Higgs doublets or certain classes of leptoquarks. We also discuss the complementarity of R_{D^{(*)}} and a measurement of the longitudinal polarization of the τ in the B→D^{*}τν decay in light of our findings.
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