  Improved upper bounds are presented on the coupling $|U_{e4}|^2$ of an
electron to a sterile neutrino $\nu_4$ from analyses of data on nuclear and
particle decays, including superallowed nuclear beta decays, the ratios
$R^{(\pi)}_{e/\mu}=BR(\pi^+ \to e^+ \nu_e)/BR(\pi^+ \to \mu^+ \nu_\mu)$,
$R^{(K)}_{e/\mu}$, $R^{(D_s)}_{e/\tau}$, and $B^+_{e 2}$ decay, covering the
mass range from MeV to GeV.
