Matrix elements of four-fermion operators in the HQET

The B-$\bar B$ mixing, B-meson lifetimes, the $B_{s}$-$\bar{B}_{s}$ lifetime difference and SUSY FCNC effects in $\Delta B=2$ processes are very important measurable quantities in B-meson phenomenology whose theoretical predictions depend on unknown matrix elements of several four-fermion operators. We present preliminary results for the matrix elements of the relevant four-fermion operators computed on a sample of 600 lattices of size $24^3\times 40$ at $\beta=6.0$, using the SW--Clover action for light quarks with rotated light quark propagators and the lattice version of the HQET for heavy quarks. As a necessary ingredient of the calculation, we also present results for the next-to-leading order matching of the full theory to the lattice HQET (one- and two-loop anomalous dimensions, one-loop QCD-HQET matching coefficients and one-loop continuum-lattice HQET matching coefficients).