Higgs singlet superfields are usually present in most extensions of the Minimal Supersymmetric Standard Model (MSSM) that address the mu-problem, such as the Next-to-Minimal Supersymmetric Standard Model (NMSSM) and the Minimal Nonminimal Supersymmetric Standard Model (MNSSM). Employing a gauge- and flavour-covariant effective Lagrangian formalism, we show how the singlet Higgs bosons of such theories can have significant contributions to B-meson flavour-changing neutral current (FCNC)

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... es for large values of tanβ>_∼ 50 at the 1-loop level. Illustrative results are presented including effects on the B_s and B_d mass differences and on the rare decay B_s→μ^+μ^-. In particular, we find that depending on the actual value of the lightest singlet pseudoscalar mass in the NMSSM, the branching ratio for B_s→μ^+μ^- can be enhanced or even suppressed with respect to the Standard Model prediction by more than one order of magnitude.