  We measure the branching fraction of the top quark to longitudinally and
right-handed polarized $W$ bosons, $F_0$ and $F_+$, using approximately 200
pb$^{-1}$ of $\bar{p}p$ collisions collected by the CDF II detector. We analyze
two quantities sensitive to the $W$ helicity: the invariant mass of the charged
lepton and the bottom-quark jet in the decay $t\to Wb \to \ell\nu b$ (where
$\ell=e$ or $\mu$), and the transverse momentum of the charged lepton. We find
$F_0=0.74^{+0.22}_{-0.34}$, and $F_+<0.27$ at the 95% confidence level. These
measurements are in agreement with the standard model predictions.
