A Hot Companion to a Blue Straggler in NGC188 as Revealed by the
  Ultra-violet Imaging Telescope (UVIT) on ASTROSAT
  We present early results from the Ultra-Violet Imaging Telescope (UVIT)
onboard the ASTROSAT observatory. We report the discovery of a hot companion
associated 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 spectral
energy distribution (SED) of the star WOCS-5885, after combining with flux
measurements from GALEX, UIT, UVOT, SPITZER, WISE and several ground-based
facilities. The resulting SED spans a wavelength range of 0.15~${\mu}$m to
7.8~${\mu}$m. This object is found to be one of the brightest FUV sources in
the cluster. An analysis of the SED reveals the presence of two components. The
cooler component is found to have a temperature of 6,000$\pm$150~K, confirming
that it is a BSS. Assuming it to be a main-sequence star, we estimate its mass
to be $\sim$ 1.1 - 1.2M$_\odot$. The hotter component, with an estimated
temperature 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 cooler
than a sub-dwarf, we speculate that it is a post-AGB/HB star that has recently
transferred its mass to the BSS, which is known to be a rapid rotator. This
binary system, which is the first BSS with a post-AGB/HB companion identified
in an open cluster, is an ideal laboratory to study the process of BSS
formation via mass transfer.
