The Effective Temperature Scale of Galactic Red Supergiants: Cool, But   Not As Cool As We Thought
  We use moderate-resolution optical spectrophotometry and the new MARCS stellar atmosphere models to determine the effective temperatures of 74 Galactic red supergiants. From these we find a new effective temperature scale that is significantly warmer than those in the literature. We show that this temperature scale, along with the newly derived bolometric corrections, gives much better agreement between our red supergiants and stellar evolutionary tracks. This agreement provides an independent verification of our new temperature scale. The combination of effective temperature and bolometric luminosities allows us to calculate stellar radii; the coolest and most luminous stars have radii of roughly 1500 solar radii (7 AU), in excellent accordance with the largest stellar radii predicted from current evolutionary theory. We find that similar results are obtained for the effective temperatures and bolometric luminosities using only the de-reddened V-K colors, providing a powerful demonstration of the self-consistency of the MARCS models. 
Emily M. Levesque, Philip Massey, K. A. G. Olsen, Bertrand Plez, Eric   Josselin, Andre Maeder, Georges Meynet