Lattice QCD Study of B-meson Decay Constants from ETMC

We discuss a lattice QCD computation of the B-meson decay constants by the ETM collaboration where suitable ratios allow to reach the bottom quark sector by combining simulations around the charm-quark mass with an exactly known static limit. The di erent steps involved in this ratio method are discussed together with an account of the assessment of various systematic e ects. A comparison of results from simulations with two and four flavour dynamical quarks is presented.