  The electron-to-nucleon ratio or electron fraction is a key parameter in many
astrophysical studies. Its value is determined by weak-interaction rates that
are based on theoretical calculations subject to several nuclear physics
uncertainties. Consequently, it is important to have a model independent way of
constraining the electron fraction value in different astrophysical
environments. Here we show that nuclear statistical equilibrium combined with
beta equilibrium can provide such a constraint. We test the validity of this
approximation in presupernova models and give lower limits for the electron
fraction in type Ia supernova and accretion-induced collapse.
