Scalar-Tensor theory as a singular subsector of Λ(φ) Plebanski gravity

It is shown that, in the absence of matter fields, the coupling of a scalar field to the non-chiral Plebanski action can be obtained by relaxing the trace component of the simplicity constraints. This is realized by considering a subclass of generalized theories, where a potential depending on invariants of the Lagrange multipliers is added to the Plebanski action. Generically such theories propagate eight degrees of freedom, but here the (singular) subclass with the potential only depending on the trace of the Lagrange multiplier is displayed as the Bergmann-Wagoner-Nordtvedt class of scalar-tensor theories.