  Strong interaction level shifts and widths in $\Sigma^-$ atoms are analyzed
by using a $\Sigma$ nucleus optical potential constructed within the
relativistic mean field approach. The analysis leads to potentials with a
repulsive real part in the nuclear interior. The data are sufficient to
establish the size of the isovector meson--hyperon coupling. Implications to
$\Sigma$ hypernuclei are discussed.
