  NGC 7789 is a $\sim$1.6 Gyr old, populous open cluster located at $\sim$2000
pc. We characterize the blue straggler stars (BSS) of this cluster using the
Ultraviolet (UV) data from the UVIT/AstroSat. We present spectral energy
distributions (SED) of 15 BSS candidates constructed using multi-wavelength
data ranging from UV to IR wavelengths. In 8 BSS candidates, a single
temperature SED is found to be satisfactory. We discover hot companions in 5
BSS candidates. The hot companions with Teff $\sim$11750-15500 K, R
$\sim$0.069-0.242 R$_{\odot}$, and L $\sim$0.25-1.55 L$_{\odot}$, are most
likely extremely low mass (ELM) white dwarfs (WDs) with masses smaller than
$\sim$0.18 M$_{\odot}$, and thereby confirmed post mass transfer systems. We
discuss the implication of this finding in the context of BSS formation
mechanisms. Two additional BSS show excess in one or more UV filters, and may
have a hot companion, however, we are unable to characterize them. We suggest
that at least 5 of the 15 BSS candidates (33%) studied in this cluster have
formed via the mass transfer mechanism.
