  Recently, a double-copy formalism was used to calculate gravitational
radiation from classical Yang-Mills radiation solutions. This work shows that
Yang-Mills theory coupled to a biadjoint scalar field admits a radiative double
copy that agrees with solutions in Einstein-Yang-Mills theory at the lowest
finite order. Within this context, the trace-reversed metric $\bar{h}^{\mu\nu}$
is a natural double copy of the gauge boson $A^{\mu a}$. This work provides
additional evidence that solutions in gauge and gravity theories are related,
even though their respective Lagrangians and nonlinear equations of motion
appear to be different.
