A Measurement of ${\cal B}(D_s \to φl^+ ν)/{cal B}(D_s\to φ π^+)$

Using the CLEO~II detector at CESR, we have measured the ratio of branching fractions ${\cal B}(D_s^+ \to \phi e^+ \nu)/{\cal B}(D_s^+ \to \phi\pi^+) = 0.54\pm 0.05\pm 0.04$. We use this measurement to obtain a model dependent estimate of ${\cal B}(D_s^+ \to \phi \pi^+)$.