LX Cyg optical, NIR and MIR spectra

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.