Study of Semileptonic Charm Decays D^0 --> pi^- ell^+ nu and D^0 --> K^-   ell^+ nu
  We investigate the decays D^0 --> pi^- l^+ nu and D^0 --> K^- l^+ nu, where l = e or mu, using approximately 7 fb^{-1} of data collected with the CLEO III detector. We find R_0 = B(D->pienu)/B(D->Kenu) = 0.082 +- 0.006 +- 0.005. Fits to the kinematic distributions of the data provide parameters describing the form factor of each mode. Combining the form factor results and R_0 gives |f^pi_+(0)|^2 |V_{cd}|^2/|f^K_+(0)|^2 |V_{cs}|^2 = 0.038 +0.006- 0.007 +0.005-0.003. 
G. S. Huang, et al. (CLEO Collaboration)