Context : Optical observations from 1989 of the Small Magellanic Cloud ( SMC ) B [ e ] supergiant star LHA 115-S 23 ( in short : S 23 ) revealed the presence of photospheric He i absorption lines , classifying S 23 as a B8 supergiant .
In our high-resolution optical spectra from 2000 , however , we could not identify any He i line .
Instead , the spectral appearance of S 23 is more consistent with the classification as an A1 supergiant , maintaining the so-called B [ e ] phenomenon .

Aims : The observed changes in spectral behaviour of S 23 lead to different spectral classifications at different observing epochs .
The aim of this research is , therefore , to find and discuss possible scenarios that might cause a disappearance of the photospheric He i absorption lines within a period of only 11 years .

Methods : From our high-resolution optical spectra , we perform a detailed investigation of the different spectral appearances of S 23 based on modern and revised classification schemes .
In particular , we derive the contributions caused by the interstellar as well as the circumstellar extinction self-consistently .
The latter is due to a partly optically thick wind .
We further determine the projected rotational velocities of S 23 in the two epochs of spectroscopic observations .

Results : Based on its spectral appearance in 2000 , we classify S 23 as A1 Ib star with an effective temperature of about 9000 K. This classification is supported by the additional analysis of the photometric UBV data .
An interstellar extinction value of E ( B - V ) \simeq 0.03 is derived .
This is considerably lower than the previously published value , which means that , if the circumstellar extinction due to the stellar wind is neglected , the interstellar extinction , and hence the luminosity of the star , are overestimated .
We further derive a rotation velocity of \varv \sin i \simeq 150 km s ^ { -1 } , which means that S 23 is rotating with about 75 % of its critical speed .
The object S 23 is thus the fourth B [ e ] supergiant with confirmed high projected rotational velocity .
The most striking result is the apparent cooling of S 23 by more than 1500 K with a simultaneous increase of its rotation speed by about 35 % within only 11 years .
Since such a behaviour is excluded by stellar evolution theories , we discuss possible scenarios for the observed peculiar variations in S 23 .

Conclusions :