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Optical observations from 1989 of the SMC B[e] supergiant star S23 revealedthe presence of photospheric HeI absorption lines. In our high-resolutionoptical spectra from 2000, however, we could not identify any HeI line. Theobserved changes in spectral behaviour of S23 lead to different spectralclassifications at different observing epochs. The aim of this research is,therefore, to find and discuss possible scenarios that might cause adisappearance of the photospheric HeI absorption lines within a period of only11 years. From our high-resolution optical spectra, we perform a detailedinvestigation of the different spectral appearances of S23. We furtherdetermine the projected rotational velocities of S23 in the two epochs ofobservations. Based on its spectral appearance in 2000, we classify S23 as A1Ibstar with an effective temperature of about 9000 K. Further, an interstellarextinction value of E(B-V) = 0.03 is derived. This is considerably lower thanthe previously published value, which means that, if the circumstellarextinction due to the stellar wind is neglected, the interstellar extinction,and hence the luminosity of the star, are overestimated. We derive a rotationvelocity of v sin i = 150km/s, which means that S23 is rotating with about 75%of its critical speed. The object S23 is thus the fourth B[e] supergiant withconfirmed high projected rotational velocity. The most striking result is theapparent cooling of S23 by more than 1500 K with a simultaneous increase of itsrotation speed by about 35% within only 11 years. Since such a behaviour isexcluded by stellar evolution theories, we discuss possible scenarios for theobserved peculiar variations in S23.

From B[e] to A[e]. On the peculiar variations of the SMC supergiant LHA 115-S 23 (AzV 172)