Characterization of the Most Luminous Star in M33: A Super Symbiotic   Binary
  We present the first spectrum of the most luminous infrared star in M33, and use it to demonstrate that the object is almost certainly a binary composed of a massive O star and a dust-enshrouded Red Hypergiant. This is the most luminous symbiotic binary ever discovered. Its radial velocity is an excellent match to that of the hydrogen gas in the disk of M33, supporting our interpretation that it is a very young and massive binary star. 
Joanna Mikolajewska, Nelson Caldwell, Michael M. Shara, and Krystian   Ilkiewicz
astro-ph.SR astro-ph.GA