  Using a more accurate effective Hamiltonian governing the time evolution in the particle-antiparticle subspace of states than the one obtained within the Lee-Oehme--Yang approach we show that in the case of particles created at the initial instant t = 0 only the masses of a stable particle and its antiparticle are the same at all t > 0 in a CPT invariant system, whereas the masses of an unstable particle and its antiparticle are equal only at t = 0 and then during their time evolution they become slightly different for times t >> 0 if CP symmetry is violated but CPT symmetry holds. This property is used to show that if the baryon number B is not conserved then the asymmetry between numbers of unstable baryons and antibaryons can arise in a CPT invariant system at t >> 0 even in the thermal equilibrium state of this system. 
A contribution to the discussion of the matter-antimatter asymmetry   problem