Improved constraint on the alpha(1) PPN parameter from lunar motion

Some theories of gravity, alternative to general relativity, introduce a preferred rest frame for local gravitational physics. At the post-Newtonian level, these preferred-frame effects in Lagrangian-based theories are described by two phenomenological parameters alpha(1) and alpha(2), of which only the latter is strongly constrained today. Following previous theoretical suggestions of Nordtvedt, Damour, and Vokrouhlicky, we show that the analysis of the lunar motion provides an improved bound of the former parameter, notably alpha(1) = (-8+/-9) X 10(-5) realistic error [and alpha(1) = (-8+/-4) X 10(-5) (90% C.L.)].