  Hoyle-Lyttleton type accretion is investigated, by taking account of not only
the effect of radiation pressure but the effect of radiation drag. We calculate
the trajectories of particles for three cases: only the effect of gravity is
considered (case A); the effect of radiation pressure is taken into account
(case B); the effect of radiation drag as well as radiation pressure is taken
into account (case C). The accretion radii for former two cases are
$2GM/v_{\infty}^2$ for case A and $2GM(1-\Gamma)/v_{\infty}^2$ for case B,
where M is the mass of the accreted object, $v_{\infty}$ the relative velocity,
and Gamma the normalized luminosity of the accreted object. We found that the
accretion radius for case C is in between those of cases A and B under the
present approximation; i.e., the accretion radius decreases due to radiation
pressure while it increases due to radiation drag. In addition, the accretion
radius for case C becomes larger as the incident velocity becomes fast. The
effect of radiation drag becomes more and more important when the velocity of
the incident particle is comparable to the light speed.
