Measuring B±→τ±ν and Bc±→τ±ν at the Z peak

The measurement of B-+/- -> tau(+/-)nu(tau) at the B factories provides important constraints on the parameter tan beta/m(H)(+) in the context of models with two Higgs doublets. Limits on this decay from e(+)e(-) collisions at the Z peak were sensitive to the sum of B-+/- -> tau(+/-)nu(tau) and B-c(+/-) -> tau(+/-)nu(tau). Because of the possibly sizeable contribution from B-c(+/-) -> tau(+/-)nu(tau) we suggest that a signal for this combination might be observed if the CERN LEP L3 Collaboration used its total data of similar to 3.6 x 10(6) hadronic decays of the Z boson. Moreover, we point out that a future linear collider operating at the Z peak (Giga Z option) could constrain tan beta/m(H)(+) from the sum of these processes with a precision comparable to that anticipated at proposed high luminosity B factories from B-+/- -> tau(+/-)nu(tau) alone.