A 5% measurement of the gravitational constant in the Large Magellanic
  Cloud
  We perform a novel test of General Relativity by measuring the gravitational
constant in the Large Magellanic Cloud (LMC). The LMC contains six well-studied
Cepheid variable stars in detached eclipsing binaries. Radial velocity and
photometric observations enable a complete orbital solution, and precise
measurements of the Cepheids' periods permit detailed stellar modelling. Both
are sensitive to the strength of gravity, the former via Kepler's third law and
the latter through the gravitational free-fall time. We jointly fit the
observables for stellar parameters and the gravitational constant. Performing a
full Markov Chain Monte Carlo analysis of the parameter space including all
relevant nuisance parameters, we constrain the gravitational constant in the
Large Magellanic Cloud relative to the Solar System to be
$G_\text{LMC}/G_\text{SS} = 0.93^{+0.05}_{-0.04}$. We discuss the implications
of this 5% measurement of Newton's constant in another galaxy for dark energy
and modified gravity theories. This result excludes one Cepheid, CEP-1812,
which is an outlier and needs further study: it is either a highly unusual
system to which our model does not apply, or it prefers
$G_\text{LMC}<G_\text{SS}$ at $2.6\sigma$. We also obtain new bounds on
critical parameters that appear in semi-analytic descriptions of stellar
processes. In particular, we measure the mixing length parameter to be
$\alpha=0.90^{+0.36}_{-0.26}$ (when assumed to be constant across our sample),
and obtain constraints on the parameters describing turbulent dissipation and
convective flux.
