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Description
The Mira variable LX Cygni (LX Cyg) showed a dramatic increase of its pulsation period in the recent decades and appears to undergo an important transition in its evolution. 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. We present optical, near- and mid-infrared 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 pulsation period is derived from AAVSO data. 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}m. An emission feature at 10.7{mu}m is attributed to SiC grains. The pulsation period of LX Cyg has increased from ~460d to ~580d within only 20 years, and is stable now. 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 oxygen- to carbon-rich stars in detail.
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