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Assuming the D̄, D, D s and B , B, B s mesons belong to triplets of SU(3) flavor symmetry, we analyse the form factors in the semileptonic decays of these mesons. Both quark and meson mass differences are taken into account. We find a number of relations, in agreement with the present data as well as with previous analyses, and predict certain ratios of form factors, not yet measured, most notably the D meson decay constant fD = 209± 39 MeV. PACS numbers(s): 13.20.-v, 13.20.Fc, 13.20.He British Royal Society Postdoctoral Fellow The B meson semileptonic decays are of special importance in our understanding of the Standard Model (see e.g. [1, 2]). Many theoretical approaches have been developed to describe these decays: heavy quark effective theory (HQET) [3]-[10], lattice calculations; e.g., [1, 11, 12], QCD sum rules; e.g., [13]-[15], the quark-hadron duality model [16], and other quark models [17][22]. The heavy quark symmetry [23], [24] relates the dynamical variables of D meson decays with the decay variables of B mesons. Using the heavy quark symmetry for the c and b quarks, the ratio of decay constants (fBs/fBd)/(fDs/fD) and the ratio of the semileptonic form factors (f + /f BK + )/(f Dπ + /f DK + ) in B and D meson decays has been estimated [4]. This approach is limited by the fact that charm and bottom quark masses, mc and mb, are, indeed, finite and there are corrections to the results of HQET that are of the order ms/mc [4, 25]. Moreover, the predictions of heavy quark effective theory are expected to be reliable only in the region of small recoil momentum (q ≈ q max). On the experimental side, there are many new results in D leptonic and semileptonic decays [26]. The CLEO experiment [27] has measured the ratio |fDπ + (0)/f + (0)| = 1.29± 0.21± 0.11, assuming pole-dominance behavior of the form factors. The decay width for Ds → μνμ has now also been measured [26]. The accumulating experimental data [26] indicate that there is a timely need to better understand the behavior of semileptonic decay form factors at q ≃ 0. Recently [25], the ratios fBs/fB and fDs/fD, as well as the double ratio (fBs/fB)/(fDs/fD) have been analysed, following the usual assumption that chiral symmetry is broken by quark mass terms in the energy density, and