|Despite being observed since the XVIIIth century, Algol systems and related objects are
still rather poorly understood. We know that they are composed by a generally B-A main sequence
star and a lighter but more evolved companion star. This paradox is explained by the transfer of mass
between the two stars, but new problems arose. In particular, I studied the mass-transfer driven spin-
up of the accreting star that drives the star to critical rotation and the puzzling, indirectly observed, non-conservative evolution.