  We study the effects of a beyond-Horndeski theory of modified gravity in the
interior of a population II star. We consider a simple phenomenological model
of a $1.1M_{\odot}$ star that has left the main sequence, has a thin Hydrogen
burning shell with a partially degenerate isothermal core, surrounded by a
radiative envelope having two regions of distinct opacities. Using suitable
matching conditions at the two internal boundaries, a numerical analysis of the
resulting stellar equations in modified gravity is carried out. While overall,
gravity may be weakened, resulting in a decrease of the luminosity and an
increase of the radius of the star, these effects are reversed near the core.
It is suggested how the model, within its limitations, can yield a bound on the
modified gravity parameter.
