# Origin of the terminology "Einstein frame" and "Jordan frame"

Consider a classical scalar-tensor theory (I'm following this paper)

$$$$S=\int d^4x\sqrt{-g}\left( \frac{1}{2\kappa^2}\mathcal{A}\left(\Phi\right)R -\frac{1}{2}\mathcal{B}\left(\Phi\right)\left(\nabla\Phi\right)^2 - V\left(\Phi\right) \right) + S_m\left[e^{2\alpha\left(\Phi\right)}g_{\mu\nu},\psi_m\right]$$$$

The Ricci scalar is $$R$$, the scalar field is $$\Phi$$, and $$\psi_m$$ are the ''matter fields''. The theory is said to be in the ''Einstein frame'' if $$\mathcal{A}=1,\mathcal{B}=1$$, and is said to be in the ''Jordan frame'' if $$\alpha=0,\mathcal{B}=1$$.

My question is: what is the origin of this terminology? It looks like the term ''Einstein frame'' was introduced by Hawking, but I can't figure out where the first paper where ''Jordan frame'' was coined (in Hawking's paper he calls it the ''Brans-Dicke frame'').