  The effect of non-zero c quark mass on b quark HQET Lagrangian, up to 1/m_b
level, is calculated at two loops. The results are expressed in terms of
dilogarithmic functions of m_c/m_b. This calculation involves on-shell two-loop
propagator-type diagrams with two different masses, m_b and m_c. A general
algorithm for reducing such Feynman integrals to the basis of two nontrivial
and two trivial integrals is constructed.
