  Yellow hypergiants represent a short-lived evolutionary episode experienced
by massive stars as they transit to and from a red supergiant phase. As such,
their properties provide a critical test of stellar evolutionary theory. The
galactic yellow hypergiant IRC +10420 is a cornerstone system for understanding
this phase, since it is the strongest post-RSG candidate known, has
demonstrated real-time evolution across the Hertzsprung-Russell diagram and
been subject to extensive mass loss. Here we report on the discovery of a twin
of IRC +10420 - IRAS 18357-0604. Optical and near-IR spectroscopy are used to
investigate the physical properties of IRAS 18357-0604, while IR photometry
probes the nature of its circumstellar environment. Pronounced spectral
similarities between IRAS 18357-0604 and IRC +10420 suggest comparable
temperatures and wind geometries, while photometric data reveals a similarly
dusty circumstellar environment, although historical mass loss appears to have
been heavier in IRC +10420. The systemic velocity implies a distance compatible
with the red supergiant dominated complex at the base of the Scutum Crux arm;
the resultant luminosity determination is consistent with a physical
association but suggests a lower initial mass than inferred for IRC +10420
(<20Msun versus ~40Msun). Evolutionary predictions for the physical properties
of supernova progenitors derived from ~18-20Msun stars - or ~12-15Msun stars
that have experienced enhanced mass loss as red supergiants - compare
favourably with those of IRAS 18357-0604, which in turn appears to be similar
to the the progenitor of SN2011dh; it may therefore provide an important
insight into the nature of the apparently H-depleted yellow hypergiant
progenitors of some type IIb SNe.
