The γ parameter in Brans-Dicke-like (light-)Scalar-Tensor theory with a universal scalar/matter coupling and a new decoupling scenario

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The post-Newtonian parameter γ resulting from a universal scalar/matter coupling is investigated in Brans-Dicke-like Scalar-Tensor theories where the scalar potential is assumed to be negligible. Conversely to previous studies, we use a perfect fluid formalism in order to get the explicit scalar-field equation. It is shown that the metric can be put in its standard post-Newtonian form. However, it is pointed out that 1-γ could be either positive, null or negative for finite value of ω₀, depending on the coupling function; while Scalar-Tensor theories without coupling always predict γ<1 for finite value of ω₀. Moreover, regardless the value of ω₀, the subclass of theories satisfying γ=1 and β=1 surprisingly leads to almost the same phenomenology as general relativity in the solar system.