The puzzling Luminous Blue Variable-like object HD 5980 in the Small Magellanic Cloud

We have observed the exceptional SMC star HD 5980 during several runs from 1989 to 1995 at ESO La Silla. CASPEC at the 3.6 m telescope and EMMI in echelle and long slit modes at NTT were used for spectroscopy. Subarcsecond images were obtained using SUSI at NTT and also an adaptive optics system at the 3.6 m telescope. In all our spectra taken before 1994 September HD 5980 shows a spectral type of WN 6. The 1994 September spectra were taken shortly after the maximum of the visual light-curve of the LBV-like phenomenon (Bateson & Jones 1994) and about one month before the observations of Barba et al. (1995). Near maximum visual brightness, HD 5980 displays a WN 11-like spectrum with the He I lines and the Balmer lines H delta and H gamma showing well-developed PCyg profiles. The sub-arcsecond images (1 ''.17 FWHM), through the near infrared bands J, H, and K, obtained in 1993 and 1996, show no stellar components down to 6.7 mag fainter than HD 5980 in K at a separation of 1 ''.0 and the 3 sigma level. For a separation of 0 ''.3 this upper bound is 4.1 mag fainter than HD 5980. The observed behavior of this object raises serious problems for our comprehension of the LBV phenomenon in the conventional scenarios of massive star evolution. The present observations cover a crucial period in the evolution of HD 5980 and will therefore be helpful for better understanding this peculiar object especially during its outburst as well as the evolution of W-R stars in general.