Companion stars are stars that orbit around each other in a binary system, where two stars are gravitationally bound to each other and orbit a common center of mass. These pairs of stars can have a variety of characteristics, such as similar or different masses, sizes, and luminosities. Studying companion stars can provide valuable information about the evolution and dynamics of stars, as well as the interactions and mass transfer between binary star systems. Additionally, companion stars can influence the properties and behavior of their partner stars, impacting phenomena such as stellar evolution, accretion processes, and the formation of planetary systems. Research on companion stars involves a range of observational and theoretical techniques, including spectroscopy, photometry, and numerical simulations. By studying companion stars, astronomers can gain insights into the fundamental processes that govern the behavior and evolution of stars in our galaxy and beyond.