We have observed 23.2±6.0−0.9+1.0 purely-leptonic decays of
Ds+−>μ+νμ from a sample of muonic one prong decay events
detected in the emulsion target of Fermilab experiment E653. Using the Ds+−>ϕμ+νμ yield measured previously in this experiment, we obtain
B(Ds+−−>μ+νμ)/B(Ds+−−>ϕμ+νμ)=0.16±0.06±0.03. In addition, we extract the decay constant fDs=194±35±20±14MeV.Comment: 15 pages including one figur