Observation of $D^0 \to \eta \omega$

Numerous singly-Cabibbo-suppressed charm meson decays are as yet unobserved. Here we report the observation of $D^0 \to \eta \omega$ in the CLEO data set. Besides the value of branching fraction of this decays in helping to measure $\eta-\eta^\prime$ mixing, it is an important background in $D^0$ Dalitz decays, such as $D^0 \to K^0_s \pi^0 \eta$ which is where we stumbled across it.