  We present a theoretical study of the experiments on coherent population
oscillations and coher- ent population trapping on the intercombination line of
174Y b. The transition involves a change of the spin and thus can not be
interpreted in terms of an effective Lambda system. The reported experiments
are done in the regime where both pump and probe fields can saturate the
transition. We demonstrate by both numerical and analytical calculations the
appearance of the interference minimum as both pump and probe start saturating
the transition. We present an analytical result for the threshold probe power
when the interference minimum can appear. We also present de- tailed study of
the appearance of the interference minimum when magnetic fields are applied.
The magnetic fields not only create Zeeman splittings but in addition make the
system open because of the couplings to other levels. We show the possibility
of interference minimum at the position of subharmonic resonances.
