  We present the first model-independent measurement of the helicity of $W$
bosons produced in top quark decays, based on a 1 fb$^{-1}$ sample of candidate
$t\bar{t}$ events in the dilepton and lepton plus jets channels collected by
the D0 detector at the Fermilab Tevatron $p\bar{p}$ Collider. We reconstruct
the angle $\theta^*$ between the momenta of the down-type fermion and the top
quark in the $W$ boson rest frame for each top quark decay. A fit of the
resulting \costheta distribution finds that the fraction of longitudinal $W$
bosons $f_0 = 0.425 \pm 0.166 \hbox{(stat.)} \pm 0.102 \hbox{(syst.)}$ and the
fraction of right-handed $W$ bosons $f_+ = 0.119 \pm 0.090 \hbox{(stat.)} \pm
0.053 \hbox{(syst.)}$, which is consistent at the 30% C.L. with the standard
model.
