Search for muon number violating decays

The SINDRUM II experiment at PSI searches for the coherent conversion of a muon into an electron in titanium. The theoretical motivation, the SINDRUM II detector and the results of the first data-taking period in 89 are presented. In total 3.2 million events have been analyzed and no candidate for the process μ-Ti→e-Ti has been found. An upper limit of Bμe<4.4·10-12 (90% C.L.) is obtained for the branching ratio. The plans to lower the sensitivity by two orders of magnitude are discussed. A muonium-antimuonium conversion experiment is being set up at PSI using the refurbished SINDRUM I detector. The goal of the experiment is to improve the sensitivity for the effective coupling constant GMbarM by a factor of ∼100