StHα -55: a Carbon Mira, not a Symbiotic Binary

We carried out a VR CI C photometric monitoring of StHα 55. In addition, we obtained low resolution absolute spectrophotometry and high resolution Echelle spectroscopy of the star. Our data show that StHα 55 is a carbon Mira, pulsating with a 395 day period, with < V > = 13.1 mean brightness and Δ V = 2.8 mag amplitude. It has a low reddening of E_B-V = 0.15, lies at a distance of 5 kpc from the Sun and 1 kpc from the Galactic plane, and its heliocentric systemic velocity is close to +22 km s^-1. The difference between the radial velocity of the optical absorption spectrum and that of the Hα emission is unusually small for a carbon Mira. The spectrum of StHα 55 can be classified as C-N5 C₂6^-. Its ¹³C/¹²C isotopic ratio is normal, and the lines of Ba II and other s-type elements, as well as Li I, have the same intensity as in field carbon stars of similar spectral type. The Balmer emission lines are very sharp and unlike those seen in symbiotic binaries. Their intensity changes in phase with the pulsation cycles in the same way as seen in field carbon Miras. We therefore conclude that StHα 55 is a bona fide, normal carbon Mira showing no feature supporting a symbiotic binary classification, which has been suggested previously.