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We present early results from the Ultra-Violet Imaging Telescope (UVIT)onboard the ASTROSAT observatory. We report the discovery of a hot companionassociated with one of the blue straggler stars (BSSs) in the old open cluster,NGC188. Using fluxes measured in four filters in UVIT's Far-UV (FUV) channel,and two filters in the near-UV (NUV) channel, we have constructed the spectralenergy distribution (SED) of the star WOCS-5885, after combining with fluxmeasurements from GALEX, UIT, UVOT, SPITZER, WISE and several ground-basedfacilities. The resulting SED spans a wavelength range of 0.15~${\mu}$m to7.8~${\mu}$m. This object is found to be one of the brightest FUV sources inthe cluster. An analysis of the SED reveals the presence of two components. Thecooler component is found to have a temperature of 6,000$\pm$150~K, confirmingthat it is a BSS. Assuming it to be a main-sequence star, we estimate its massto be $\sim$ 1.1 - 1.2M$_\odot$. The hotter component, with an estimatedtemperature of 17,000$\pm$500~K, has a radius of $\sim$ 0.6R$_\odot$ and L$\sim$ 30L$_\odot$. Bigger and more luminous than a white dwarf, yet coolerthan a sub-dwarf, we speculate that it is a post-AGB/HB star that has recentlytransferred its mass to the BSS, which is known to be a rapid rotator. Thisbinary system, which is the first BSS with a post-AGB/HB companion identifiedin an open cluster, is an ideal laboratory to study the process of BSSformation via mass transfer.

A Hot Companion to a Blue Straggler in NGC188 as Revealed by the Ultra-violet Imaging Telescope (UVIT) on ASTROSAT