Dominant type of drag force (gravitational versus hydrodynamic) as a function of the semi-major axis and the mass ratio for a secondary with M₂ = 1 M_⊙. The red dashed line indicates the radius of the primary (R₁) at the ZAMS. The dotted and dash-dotted lines refer to the Roche radii of the primary at a semi-major axis of 3 R_⊙ and 8 R_⊙, respectively. If the radius exceeds the size of the Roche lobe, the system should not exist.