Abstract

Ultracompact white dwarf binaries or AM CVns have orbits of less than 70 min. They are particularly interesting because they are sources of low-frequency gravitational waves and are laboratories for studying mass accretion under extreme conditions. A fraction of these binaries show outbursts, but the processes behind these accretion related events are not fully understood. In this talk I will discuss what we know on accretion in AM CVns, focusing on the recent discoveries of outbursts in systems with orbital periods longer than 50 min, a period range where the existence and properties of outbursts challenge the commonly invoked disk instability model. Instead, the peculiar characteristics of outbursts in these long-period systems, and their comparison to other accreting white dwarfs, have proved the existence of other mechanisms. During the talk I will also address the importance of considering these recently identified processes in order to quantify their influence on AM CVn evolution, and the impact they might have on the detection rate of AM CVns with upcoming surveys and gravitational wave observatories such as LISA.