Low-mass contact binaries as probes of stellar merging events

Gravitational interaction is important for nearly every aspect of Astrophysics. Theoretical models are tough tested in cases where strong, stellar winds, angular momentum loss, and mass transfer take place in the course of stellar evolution within a common envelope. Stellar interaction impacts the formation and evolution of diverse types of binary and multiple stellar systems. Therefore, evolution in a binary (and a multiple) stellar environment is still an open question in Astrophysics while contact binaries are challenging the well-established and solid theories that concern thermal equilibrium and orbital stability. There is a vast number of such systems observed in the Milky Way and the Magellanic Clouds. Systematic (independent or combined) observations, all-sky surveys, ground-based and space telescopes have offered very detailed photometric and spectroscopic information about these systems. Special attention is given when extreme cases are observed, such as ultra-short orbital period contact binaries or extremely low mass ratio systems. These cases can give insights into the physical and orbital parameters of low-mass contact binaries and their temporal variations. For example, one can investigate the orbital period modulation, the spot activity, and the possibility of contact binaries to host planets, as well as predict a stellar merger, red nova events, and a possible connection with exotic stellar populations such as blue stragglers. The purpose of this work is to present the recent developments in observations of low-mass contact binaries, highlighting the most important findings, in combination with existing theories. The research of such systems, at an international level, is constantly challenging our current knowledge of stellar interactions and advances our understanding of stellar dynamics in common envelope and contact phases.