# Common envelope: progress and transients

## TL;DR

This paper reviews recent advances in understanding common envelope physics, emphasizing the role of recombination energy in envelope ejection and its observational signatures, including links to luminous red novae and irregular variables.

## Contribution

It highlights the significance of recombination energy in different common envelope ejection scenarios and discusses their observational implications.

## Key findings

- Recombination energy enables complete envelope ejection in prompt binary formation.
- Different effects of recombination energy lead to a tenfold variation in efficiency between fast and slow spiral-ins.
- Common envelope events are linked to luminous red novae and luminous irregular variables.

## Abstract

We review the fundamentals and the recent developments in understanding of common envelope physics. We report specifically on the progress that was made by the consideration of the recombination energy. This energy is found to be responsible for the complete envelope ejection in the case of a prompt binary formation, for the delayed dynamical ejections in the case of a self-regulated spiral-in, and for the steady recombination outflows during the transition between the plunge-in and the self-regulated spiral-in. Due to different ways how the recombination affects the common envelope during fast and slow spiral-ins, the apparent efficiency of the orbital energy use can be different between the two types of spiral-ins by a factor of ten. We also discuss the observational signatures of the common envelope events, their link a new class of astronomical transients, Luminous Red Novae, and to a plausible class of very luminous irregular variables.