QCD Corrected $1/m_b$ Contributions to $B\bbar$--Mixixng
  We calculate the QCD corrected effective Hamiltonian for $B\bbar$--Mixing in heavy quark effective theory including corrections of the order $\Lambda_{QCD} / m_b$. The matrix elements of the subleading operators are estimated using the vacuum insertion assumption. We show that the major part of the subleading corrections may be absorbed into the heavy meson decay constant $f_B$; the remaining corrections are only due to QCD effects and give an enhancement of $\Delta M$ of 5\%. 
Wolfgang Kilian and Thomas Mannel