Common envelope evolution

Many binary star systems are not wide enough to contain the progenitor stars from which they were made. One explanation for this is that when one star becomes a red giant a common envelope forms around both stars in the binary system. The core of the giant and its companion star continue to orbit one another inside the envelope. Frictional energy deposited into the common envelope may lead to its ejection and, if so, a close binary system is formed from the core of the former giant star and its relatively untouched companion. When the primary is an asymptotic giant branch star the core becomes a hot carbon-oxygen white dwarf which may ionise the ejected envelope and illuminate a planetary nebula. Many other types of binary systems form through common envelope evolution such as low-mass X-ray binaries and cataclysmic variables. In the case of a failed envelope ejection when the cores merge, rapidly-rotating solitary giants similar to FK Comae stars form. In this short review we focus on attempts to constrain parameters of common envelope evolution models and also describe the latest efforts to model this elusive phase of binary stellar evolution.