First measurement of the form factors in $D\to \rho e^+ \nu_e$

Using a 281 pb$^{-1}$ data sample collected with the CLEO-c detector at the $\psi(3770)$ resonance, we present improved measurements of absolute branching fractions in the semileptonic decays $D^0 \to \rho^- e^+ \nu_e$ and $D^+ \to \rho^0 e^+ \nu_e$. By performing a four-dimensional maximum likelihood fit to the distribution of kinematic variables, we have measured, for the first time, the semileptonic form factors in these modes. An extension of the analysis with the $\approx 800$~pb$^{-1}$ complete data set is also discussed.