  We calculate the amount of angular momentum that thermal photons carry out of
a viscous black hole accretion disc, due to the strong Doppler shift imparted
to them by the high orbital velocity of the radiating disc material. While the
emission of radiation can not drive accretion on its own, we find that it does
result in a loss of specific angular momentum, thereby contributing to an
otherwise viscosity-driven accretion flow. In particular, we show that the
fraction of the angular momentum that is lost to thermal emission at a radius r
in a standard, multi-color disc is ~ 0.4r_s/r, where r_s is the Schwarzschild
radius of the black hole. We briefly highlight the key similarities between
this effect and the closely related Poynting-Robertson effect.
