  We study coherent population oscillations (CPO) in an odd isotope of the
two-electron atom Yb. The experiments are done using magnetic sublevels of the
$ F_g = 5/2 \rightarrow F_e = 3/2 $ hyperfine transition in $^{173}$Yb of the $
{\rm {^1S_0} \rightarrow {^3P_1}} $ intercombination line. The experiments are
done both with and without an appied magnetic field. In the absence of an
applied field, the complicated sublevel structure along with the saturated
fluorescence effect causes the linewidth to be larger than the 190 kHz natural
linewidth of the transition. In the presence of a field (of magnitude 330 mG),
a well-defined quantization axis is present which results in the formation of
two M-type systems. The total fluorescence is then limited by spin coherence
among the ground sublevels. In addition, the pump beam gets detuned from
resonance which results in a reduced scattering rate from the $ {\rm ^3P_1} $
state. Both of these effects result in a reduction of the linewidth to a
subnatural value of about 100 kHz.
