Measurement of the form-factor ratios for

The form factor ratios r_V=V(0)/A₁(0), r₂=A₂(0)/A₁(0) and r₃=A₃(0)/A₁(0) in the decay , have been measured using data from charm hadroproduction experiment E791 at Fermilab. From 3034 (595) signal (background) events in the muon channel, we obtain r_V=1.84±0.11±0.09, r₂=0.75±0.08±0.09 and, as a first measurement of r₃, we find 0.04±0.33±0.29. The values of the form factor ratios r_V and r₂ measured for the muon channel are combined with the values of r_V and r₂ that we have measured in the electron channel. The combined E791 results for the muon and electron channels are r_V=1.87±0.08±0.07 and r₂=0.73±0.06±0.08.