UOCS: X. Rich collection of post-mass-transfer systems in NGC 6791

Aims. NGC 6791 is one of the richest old open clusters in the Milky Way. Its position above the Galactic plane and its number density make it an interesting middle ground between Galactic open and globular clusters. We aim to detect the UV-bright population of NGC 6791 using AstroSat/UVIT images in near-UV and far-UV filters and characterise the known post-mass-transfer systems such as blue straggler stars (BSSs). Methods. We identified 20 members with large UV flux (out of 91 cluster members among 1180 detections) – which is suggestive of binarity, interactions, or stellar activity – using a multi-wavelength spectral energy distribution analysis. Results. We characterised 62 isolated cluster members, including five hot subdwarfs (sdA/sdB). Additionally, we detected ten sdA/sdB/extremely low-mass (ELM) white dwarf-type candidates hidden alongside other cluster members. Additionally, we report the discovery of four candidate blue lurkers, which are main sequence stars with mass accretion history. Conclusions. We report that this cluster has a variety of stellar (pre-)remnants, such as sdBs, sdAs, and ELM white dwarfs, which are by-products of binary evolution. These are likely to be post-mass-transfer binaries found throughout the evolutionary phases from the main sequence to the post-horizontal branch. Therefore, this dynamically old open cluster is unique, making it an ideal test bed for dynamical studies.