  The processes $ e^+e^-\rightarrow \Sigma^0\overline{\Sigma}{}^0 $ and $
e^+e^-\rightarrow\Sigma^+\overline{\Sigma}{}^-$ are studied using
initial-state-radiation events in a sample of 980 $\,\mbox{fb}^{-1}$ collected
with the Belle detector at the KEKB asymmetric-energy $ e^+e^- $ collider. The
cross sections from the mass threshold to $ 3{\mathrm{\,Ge\kern -0.1em
V\!/}c^2} $ and the effective form factors of $ \Sigma^0 $ and $ \Sigma^+ $ are
measured. In the charmonium region, we observe the decays
$J/\psi\rightarrow\Sigma^0\overline{\Sigma}{}^0$ and
$J/\psi\rightarrow\Sigma^+\overline{\Sigma}{}^-$ and determine the respective
branching fractions.
