  Based on the results of accurate variational calculations we demonstrate
stability of the five-body negatively charged ions $a^{+} b^{+} \mu^{-} e^{-}
e^{-}$. Each of these five-body ions contains two electrons $e^{-}$, one
negatively charged muon $\mu^{-}$ and two nuclei of the hydrogen isotopes $a, b
= (p, d, t)$. The bound state properties of these five-body ions, including
their hyperfine structure, are briefly discussed. We also investigate the
hyperfine structure of the ground states of the four-body muonic quasi-atoms
$a^{+} b^{+} \mu^{-} e^{-}$. In particular, we determine the hyperfine
structure splittings for the ground state of the four-body muonic quasi-atoms:
$p^{+} d^{+} \mu^{-} e^{-}$ and $p^{+} t^{+} \mu^{-} e^{-}$.

Atomic Physics;Atomic and Molecular Clusters