An unprecedented change in the spectrum of S Doradus: As cool as it gets

Recent observations of S Doradus, the archetype of luminous blue variables (LBVs), reveal that the star's optical spectrum now resembles an F-type sup ergiant, with a rich complex of absorption lines. Despite nearly 50 years o f spectroscopic monitoring, such a spectrum has never before been seen for S Dor despite numerous occasions when the star was equally bright. However, such F-type spectra have been seen in other LBVs, including Var B in M33 d uring a recent outburst, and in eta Carina during an outburst in 1893. The singly ionized metal lines arise in a layer moving away from the star (towa rd us) at 50 km s(-1), consistent with the lines forming in a " pseudophoto sphere " originating in the stellar wind. The temperature suggested by the F-type spectrum is as cool as an LBV can get. Our CCD UBV photometry shows that the star has brightened only 0.3 mag in V since 1996, at which time it showed an emission-line spectrum characteristic of LBVs at minimum. The de reddened B - V color is consistent with an effective temperature suggested by the spectrum (7500 K), but there is a pronounced UV excess as evidenced by U-B. Why the behavior of S Dor has suddenly changed remains a mystery.