Past orbital evolution of the system assuming present-day mass ratios of q = 12, 15 and 18. During mass transfer phases time increases towards the right in this figure. For each curve the right-most point represents the currently observed values, while the left-most point corresponds to an arbitrary choice of q = 1/3. Circles are a selection of detached and semidetached systems from the S​_B⁹ catalogue. Crosses are systems from the same catalogue for which it is unclear if the binary has undergone a mass ratio inversion, so two points are included per system with inverse values for q (see text for details).