  In the summer and fall of 2000 the yellow hypergiant Rho Cassiopeiae dimmed
by more than a visual magnitude, while its effective temperature decreased from
\~7000 K to below 4000 K over ~200 d. We observed the highest mass-loss rate of
  ~5 % of the solar mass per year in a single stellar eruption so far (Lobel et
al. 2003, ApJ, 583, 923). We determine from newly formed TiO bands a gas
mass-loss rate of M_dot ~ 5.4 10^{-2} Msun/yr, which is of the same order of
magnitude as has been proposed for the giant outbursts of the Luminous Blue
Variable Eta Carinae. Over the past two years since the outburst event we
observe a very prominent inverse P Cygni profile in Balmer H alpha. Strong
Halpha emission has not before been observed in the hypergiant over this long
period of time, signaling an unusual strong collapse of the upper hydrogen
atmosphere, which we also observed in the months before the 2000 outburst.
Since the brightness decrease of March 2003 we observe a transformation of the
H alpha profile into a P Cygni profile, signaling a strong expansion of Rho
Cas's upper atmosphere, which could possibly result in a new outburst event.
High-resolution spectroscopic observations reveal that the lower photosphere
rapidly expanded until May-June 2003. Very recent high-resolution observations
of September 2003 show however that the photospheric absorption lines did not
shift far blueward as observed in July 2000 before the strong brightness
decrease of the outburst. The Fe I 5572 A is redshifted, signaling the collapse
of the lower photosphere. A new strong brightness decrease by more than a
magnitude in V for the fall and winter of 2003 is therefore not expected.
