Context : SAO 244567 , the exciting star of the Stingray nebula , is rapidly evolving .
Previous analyses suggested that it has heated up from an effective temperature of about 21 kK in 1971 to over 50 kK in the 1990s .
Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star .

Aims : A comprehensive model-atmosphere analysis of UV and optical spectra taken during 1988 – 2013 should reveal the detailed temporal evolution of its atmospheric parameters and provide explanations for the unusually fast evolution .

Methods : Fitting line profiles from static and expanding non-LTE model atmospheres to the observed spectra allowed us to study the temporal change of effective temperature , surface gravity , mass-loss rate , and terminal wind velocity .
In addition , we determined the chemical composition of the atmosphere .

Results : We find that the central star has steadily increased its effective temperature from 38 kK in 1988 to a peak value of 60 kK in 2002 .
During the same time , the star was contracting , as concluded from an increase in surface gravity from \log g \hskip { -1.422638 pt } = \hskip { -1.422638 pt } 4.8 to 6.0 and a drop in luminosity .
Simultaneously , the mass-loss rate declined from \log ( \dot { M } / M _ { \odot } yr ^ { -1 } ) = -9.0 to -11.6 and the terminal wind velocity increased from v _ { \infty } = 1800 km/ s to 2800 km/ s. Since around 2002 , the star stopped heating and has cooled down again to 55 kK by 2006 .
It has a largely solar surface composition with the exception of slightly subsolar carbon , phosphorus , and sulfur .
The results are discussed by considering different evolutionary scenarios .

Conclusions : The position of SAO 244567 in the log T _ { \mathrm { eff } } – \log g plane places the star in the region of sdO stars .
By comparison with stellar-evolution calculations , we confirm that SAO 244567 must be a low-mass star ( M < 0.55 M _ { \odot } ) .
However , the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years .
We speculate that the star could be a late He-shell flash object .
Alternatively , it could be the outcome of close-binary evolution .
Then SAO 244567 would be a low-mass ( 0.354 M _ { \odot } ) helium prewhite dwarf after the common-envelope phase , during which the planetary nebula was ejected .