Effect of H^pm on B^pmto τ^pmν_τ and D^pm_sto μ^pmν_μ,τ^pmν_τ

The recent observation of the purely leptonic decay B^\pm\to \tau^\pm\nu_\tau at the B factories permits a sizeable contribution from a charged Higgs boson (H^\pm). Such a H^\pm would also contribute to the decays D^\pm_s\to \mu^\pm\nu_\mu and D^\pm_s\to \tau^\pm\nu_\tau, which are being measured with increasing precision at CLEO-c. We show that the branching ratios of D^\pm_s\to \mu^\pm\nu_\mu and D^\pm_s\to \tau^\pm\nu_\tau could be suppressed by up to 10% from the Standard Model prediction, which is larger than the anticipated precision in the measurements of these decays at forthcoming BES-III.