  In this review lifetime measurements of ions with at most two electrons are
summarized. Such highly ionized systems have been studied - until now - only in
the Experimental Storage Ring of the GSI in Darmstadt. Emphasis is put on
decays via the weak interaction. The first observations of beta-decay into
bound atomic states are described as well as its time mirrored counterpart, the
electron-capture decay. In the latter case the decays of hydrogen- and
helium-like ions are compared with a surprising result. Further on, the
observation of sinusoidal modulations of the decay rate in two-body decays is
summarized. As a possible cause an interference due to the emission of
neutrinos with different rest mass is discussed.
