  We compute the strangeness-conserving double beta decay of $\Sigma^-$
hyperons, which is the only hadronic system that can undergo such decays. We
consider both, the lepton number conserving %$\Sigma^-\go \Sigma^++2e^-+2
\bar{\nu}$ $\Sigma^-\go \Sigma^+e^-e^-\bar{\nu}\bar{\nu}$
($\beta\beta_{\Sigma_{2\nu}}$) and the lepton number violating $\Sigma^-\go
\Sigma^+e^-e^-$ ($\beta\beta_{\Sigma_{0\nu}}$) modes. The branching ratios of
these $\beta\beta$ decays are suppressed at the level of $10^{-30}$ considering
a light neutrino scenario in the case of the $\beta\beta_{\Sigma_{0\nu}}$
channel. The dynamical origin of such low rates and their possible enhancements
are briefly discussed. Given its simplicity those decays can be used also for
the purposes of illustrating the main features of double beta decays.
