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SwSt 1 (PN G001.5-06.7) is a bright and compact planetary nebula containing alate [WC]-type central star. Previous studies suggested that the nebular andstellar lines are slowly changing with time. We studied new and archivaloptical and ultraviolet spectra of the object. The [OIII] 4959 and 5007 A to$\mathrm{H}\beta$ line flux ratios decreased between about 1976 and 1997/2015.The stellar spectrum also shows changes between these epochs. We modeled thestellar and nebular spectra observed at different epochs. The analyses indicatea drop of the stellar temperature from about 42 kK to 40.5 kK between 1976 and1993. We do not detect significant changes between 1993 and 2015. Theobservations show that the star performed a loop in the H-R diagram. This ispossible when a shell source is activated during its post-AGB evolution. Weinfer that a late thermal pulse (LTP) experienced by a massive post-AGB starcan explain the evolution of the central star. Such a star does not expandsignificantly as the result of the LTP and does not became a born-again redgiant. However, the released energy can remove the tiny H envelope of the star.

The cooling-down central star of the planetary nebula SwSt\,1: a late thermal pulse in a massive post-AGB star?