  We carried out a VRI photometric monitoring of StHa 55, and in addition we
obtained low resolution absolute spectro-photometry and high resolution Echelle
spectroscopy. Our data show that StHa 55 is a carbon Mira, pulsating with a 395
day period, with <V>=13.1 mean brightness and 2.8 mag amplitude in V band. It
suffers from a low reddening (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/sec. The difference between the radial velocity of
the optical absorption spectrum and that of the Ha emission is unusually small
for a carbon Mira. The spectrum of StHa 55 can be classified as C-N5 C2 6-. Its
13C/12C isotopic ratio is normal, and lines of BaII and other s-type elements,
as well as LiI, 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 StH 55 is a bona fide, normal carbon Mira showing no feature supporting a
symbiotic binary classification, as previously hypothesized.
