  NGC 6791 is one of the richest old open clusters in the Milky Way. Its
position above the Galactic plane and the number density makes it an
interesting middle ground between Galactic open and globular clusters. We aim
to detect the UV bright population of NGC 6791 using \textit{AstroSat}/UVIT
images in near-UV and far-UV filters and characterise the known post mass
transfer systems such as blue straggler stars (BSSs). We identified 20 members
with large UV flux (out of 91 cluster members among 1180 detections),
suggestive of binarity, interactions or stellar activity using multi-wavelength
spectral energy distribution analysis. We characterised 62 isolated cluster
members, including five hot subdwarfs (sdA/sdB). Additionally, we detected ten
sdA/sdB/extremely low mass white dwarf (ELM) 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. We
report that this cluster has a variety of stellar (pre-)remnants, such as sdBs,
sdAs, and ELM white dwarfs, that are by-products of binary evolution. The above
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 testbed for
dynamical studies.
