  The massive post-Main Sequence star W243 in the galactic starburst cluster
Westerlund 1 has undergone a spectral transformation from a B2Ia supergiant
devoid of emission features in 1981 to an A-type supergiant with a rich
emission-line spectrum by 2002/03. We used VLT/UVES and VLT/FLAMES to obtain
high-resolution spectra on six epochs in 2003/04 (UVES) and ten epochs in
2008/09 (FLAMES). These spectra are used alongside other VLT/FLAMES and
NTT/EMMI spectra to follow the evolution of W243 from 2002 to 2009. W243
displays a complex, time-varying spectrum with emission lines of Hydrogen,
Helium and Lyman-pumped metals, forbidden lines of Nitrogen and Iron, and a
large number of absorption lines from neutral and singly-ionized metals. Many
lines are complex emission/absorption blends. The LBV has a current temperature
of ~8500K (spectral type A3Ia+), and displays signs of photospheric pulsations
and weak episodic mass loss. Nitrogen is highly overabundant, with Carbon and
Oxygen depleted, indicative of surface CNO-processed material and considerable
previous mass-loss, although current time-averaged mass-loss rates are low. The
emission-line spectrum forms at large radii, when material lost by the LBV in a
previous mass-loss event is ionized by an unseen hot companion. Monitoring of
the near-infrared spectrum suggests that the star has not changed significantly
since it finished evolving to the cool state, close to the Humphreys-Davidson
limit, in early 2003. [ABRIDGED]
