  We consider contributions of non-standard tbW effective operators to the
decay of an unpolarized top quark into a bottom quark and a W gauge boson at
next-to-leading order in QCD. We find that the dipole operator O_{LR}
contribution to the transverse-plus W helicity fraction F_+ is significantly
enhanced compared to the leading order result at non-vanishing bottom quark
mass. Nonetheless, presently the most sensitive observable to direct O_{LR}
contributions is the longitudinal W helicity fraction F_L. In particular, the
most recent CDF measurement of F_L already provides the most stringent upper
bound on O_{LR} contributions, even when compared with indirect bounds from the
rare decay B -> X_s gamma.
