Evolutionary route leading to the formation of the 38.9 M_⊙ + 1.3 M_⊙ (q = 0.033) BH-NS merger for metallicity Z = 0.0001 (model M280). The evolution from ZAMS to the formation of the BH-NS system takes ∼30.4 Myr, and then it takes ∼2.29 Gyr for the system to in-spiral (angular momentum loss due to the emission of gravitational waves), leading to the final merger of the two compact objects. (CHeB: core helium burning star, He star: naked helium star, HG: Hertzsprung gap star). No tidal spin-up is expected due to large orbital separations when binary components are naked helium stars: a ∼ 25 R_⊙ (P_orb = 2.1d; star A) and a ∼ 200 R_⊙ (P_orb = 60d; star B; see Sect. 2.1).