First measurement of [Gamma]([ital D][sub [ital s]][sup +][r arrow][mu][sup +][nu])/[Gamma]([ital D][sub [ital s]][sup +][r arrow][phi][pi][sup +])

We present a measurement of [Gamma]([ital D][sub [ital s]][sup +][r arrow][mu][sup +][nu])/[Gamma]([ital D][sub [ital s]][sup +][r arrow][phi][pi][sup +])= 0.245[plus minus]0.052[plus minus]0.074. Using this ratio and [ital scrB]([ital D][sub [ital s]][sup +][r arrow][phi][pi][sup +])=(3.7[plus minus]0.9)% we extract [ital f][sub [ital D][ital s]]=344[plus minus]37[plus minus]52[plus minus]42 MeV, where the last error is due to the uncertainty in the [ital D][sub [ital s]][sup +][r arrow][phi][pi][sup +] absolute branching ratio. This result is larger than most theoretical predictions.