  Scalar-tensor theories of gravity can lead to modifications of the
gravitational force inside astrophysical objects. We exhibit that compact stars
such as white dwarfs provide a unique set-up to test beyond Horndeski theories
of ${\rm G}^3$ type. We obtain stringent and independent constraints on the
parameter $\Upsilon$ characterizing the deviations from Newtonian gravity using
the mass-radius relation, the Chandrasekhar mass limit and the maximal
rotational frequency of white dwarfs. We find that white dwarfs impose stronger
constraints on $\Upsilon$ than red and brown dwarfs.
