The cooling-down central star of the planetary nebula SwSt\,1: a late
  thermal pulse in a massive post-AGB star?
  SwSt 1 (PN G001.5-06.7) is a bright and compact planetary nebula containing a
late [WC]-type central star. Previous studies suggested that the nebular and
stellar lines are slowly changing with time. We studied new and archival
optical 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 the
stellar and nebular spectra observed at different epochs. The analyses indicate
a drop of the stellar temperature from about 42 kK to 40.5 kK between 1976 and
1993. We do not detect significant changes between 1993 and 2015. The
observations show that the star performed a loop in the H-R diagram. This is
possible when a shell source is activated during its post-AGB evolution. We
infer that a late thermal pulse (LTP) experienced by a massive post-AGB star
can explain the evolution of the central star. Such a star does not expand
significantly as the result of the LTP and does not became a born-again red
giant. However, the released energy can remove the tiny H envelope of the star.
