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Binary systems consisting of a secondary accreting form a wind-emittingprimary are ubiquitous in astrophysics. The phenomenology of suchBondi-Hoyle-Lyttleton (BHL) accretors is particularly rich when an accretiondisc forms around the secondary. The outer radius of such discs is commonlyestimated from the net angular momentum produced by a density variation ofmaterial across the BHL or Bondi accretion cylinder, as the latter is tiltedwith respect to the direction to the primary due to orbital motion. But thisapproach has ignored the fact that the wind experiences an outward drivingforce that the secondary does not. In actuality, the accretion stream fallstoward a retarded point in the secondary's orbit as the secondary is pulledtoward the primary relative to the stream. The result is a finite separation or"accretion stream impact parameter" (ASIP) separating the secondary and stream.When the orbital radius a_o exceeds the BHL radius r_b, the ratio of outer discradius estimated as the ASIP to the conventional estimatea_o^{1/2}/r_b^{1/2}>1. We therefore predict that discs will form at largerradii from the secondary than traditional estimates. This agrees with theimportance of the ASIP emphasized by Huarte-Espinosa et al. (2013) and thepractical consequence that resolving the initial outer radius of such anaccretion disc in numerical simulations can be less demanding than what earlierestimates would suggest.

Size of discs formed by wind accretion in binaries can be underestimated if the role of wind-driving force is ignored