  Context: The Mira variable LX Cyg showed a dramatic increase of its pulsation
period in the recent decades and appears to undergo an important transition in
its evolution. Aims: We aim at investigating the spectral type evolution of
this star over the recent decades as well as during one pulsation cycle in more
detail and discuss it in connection with the period evolution. Methods: We
present optical, near- and mid-IR low-resolution as well as optical
high-resolution spectra to determine the current spectral type. The optical
spectrum of LX Cyg has been followed for more than one pulsation cycle. Recent
spectra are compared to archival spectra to trace the spectral type evolution
and a Spitzer mid-IR spectrum is analysed for the presence of molecular and
dust features. Furthermore, the current period is derived from AAVSO data.
Results: It is found that the spectral type of LX Cyg changed from S to C
sometime between 1975 and 2008. Currently, the spectral type C is stable during
a pulsation cycle. It is shown that spectral features typical of C-type stars
are present in its spectrum from ~0.5 to 14 $\mu{\rm{m}}$. An emission feature
at 10.7 $\mu{\rm{m}}$ is attributed to SiC grains. The period of LX Cyg has
increased from ~460 d to ~580 d within only 20 years, and is stable now.
Conclusions: We conclude that the change in spectral type and the increase in
pulsation period happened simultaneously and are causally connected. Both a
recent thermal pulse (TP) and a simple surface temperature decrease appear
unlikely to explain the observations. We therefore suggest that the underlying
mechanism is related to a recent third dredge-up mixing event that brought up
carbon from the interior of the star, i.e. that a genuine abundance change
happened. We propose that LX Cyg is a rare transition type object that is
uniquely suited to study the transformation from O- to C-rich stars in detail.
