We give a review and present a comprehensive calculation for the leptonic constant B_c of the low-lying pseudoscalar and vector states of B_c-meson in the framework of static and QCD-motivated nonrelativistic potential models taking into account the one-loop and two-loop QCD corrections in the short distance coefficient that governs the leptonic constant of B_c quarkonium system. Further, we use the scaling relation to predict the leptonic constant of the nS-states of the (b_bar)c system. Our

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... sults are compared with other models to gauge the reliability of the predictions and point out differences.