  The concept of weak invariant is introduced. Then, the weak invariants
associated with time-dependent quantum dissipative systems are discussed in the
context of master equations of the Lindblad type. In particular, with the help
of the su(1,1) Lie-algebraic structure, the weak invariant is explicitly
constructed for the quantum damped harmonic oscillator with the time-dependent
frequency and friction coefficient. This generalizes the Lewis-Riesenfeld
invariant to the case of nonunitary dynamics in the Markovian approximation.
