2016A&A...585A.145U

The Mira variable LX Cygni (LX Cyg) has shown a dramatic increase of its pulsation period in the recent decades and is appearing to undergo an important transition in its evolution. We aim to investigate the spectral type evolution of this star over 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. We compare recent spectra to archival spectra to trace the spectral type evolution, and we analyse a Spitzer mid-IR spectrum for the presence of molecular and dust features. Furthermore, the current pulsation period is derived from AAVSO data. We 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. We show that spectral features typical of C-type stars are present in its spectrum from ∼0.5 to 14µm, and attribute an emission feature at 10.7µm to SiC grains. Within only 20yr, the pulsation period of LX Cyg has increased from ∼460d to ∼580d and is stable now. We conclude that the change in spectral type and increase in pulsation period happened simultaneously and are causally connected. Both a recent thermal pulse 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.