Contact-binary Evolution with Energy Transfer and Saturated Magnetic Braking

Abstract The evolution of low-mass contact binaries is influenced by angular momentum loss, mass and energy transfer (ET), and the nuclear evolution of the components. They have periods shorter than 1 day, and we expect their period evolution to be dominated by magnetic braking (MB). Evidence for saturated MB was presented by studying the period distribution of detached eclipsing binaries. This means the strength of MB likely does not cause a steep period-shrinking relation derived from the widely used Skumanich law. We find further evidence for saturated MB by considering evolutionary models of low-mass contact binaries. We also show that ET must play an important role over a wide parameter range in producing the observed low mass ratios of contact binaries.