Typical temporal mass variation of a binary star calculated with the Brussels binary evolution code. The shown variations are for an initial 9.0 + 5.4 M_⊙ binary with an orbital period of 2.25 days. The black line is the mass variation of the initially most massive star (the mass loser) and the gray line is the initially least massive star (the mass gainer). The slow Algol phase is highlighted in thick red.