Binaries, blue stragglers, stripped stars, their companions - characteristics and evolutionary pathways

Most stars belong to binary systems, characterized by a present-day binary population (PDBP), which includes properties such as period, mass ratio, eccentricity distributions, and the multiplicity fraction. Observational incompleteness and limited statistics pose challenges, yet cluster PDBP provides valuable constraints on the initial conditions. In the first part, we will discuss the binary population of star clusters and their tidal tails. Both clusters and tails have binary fractions of 0.3–0.6, though predicted differences in period and mass distributions are likely obscured by stochasticity and small samples. The second part will focus on the binary products in open clusters. Binary evolution can lead to interactions and the formation of blue straggler stars, which offer crucial insight into binary interactions. Ultraviolet imaging with UVIT/AstroSat revealed diverse hot companions to blue stragglers, including hot subdwarfs and extremely low mass to high mass white dwarfs, pointing to varied mass-transfer histories and evolutionary status in cluster environments. In a related investigation, we studied the population of Be stars in the young open cluster NGC 663, identifying new candidates and characterising their UV and optical properties. These rapidly rotating, mass-losing stars may represent an accretor in a binary, with the companion resulting in a striping star. Together, these studies provide a unified view of blue stragglers, stripped stars, and their companions, offering new constraints on binary evolution in open clusters. Our results emphasise the unique capability of UV observations to unravel complex stellar histories and reveal otherwise hidden binary products.