Recently there has been interest in the correlation between R(D*) and the
branching ratio (BR) of Bc​→τν in models with a charged scalar
H^\pm. Any enhancement of R(D*) by H± alone (in order to agree with
current data) also enhances BR(Bc​→τν), for which there has been no
direct search at hadron colliders. We show that LEP data taken at the Z peak
requires BR(Bc​→τν) < 10%, and this constraint is significantly
stronger than the recent constraint BR(Bc​→τν) < 30% from
considering the lifetime of B_c. In order to respect this new constraint, any
explanation of the R(D) and R(D*) anomaly in terms of H± alone would
require the future measurements of R(D*) to be even closer to the Standard
Model prediction. A stronger limit on BR(Bc​→τν) (or its first
measurement) would be obtained if the L3 collaboration used all its data taken
at the Z peak.Comment: 14 pages, 4 figures, a reference and two sentences adde