Astrometry and photometry for two dwarf carbon stars

Preliminary trigonometric parallaxes and BVI photometry are presented for two dwarf carbon stars, LP 765-18 (=LHS 1075) and LP 328-57 (=CLS 96). The data are combined with the literature values for a third dwarf carbon star, G77-61 (=LHS 1555). All three stars have very similar luminosities (9.6 < M-V < 10.0) and very similar broadband colors across the entire visual-to-near-IR (BVIJHK) wavelength range. Their visual (BVI) colors differ from all known red dwarfs, subdwarfs, and white dwarfs. In the M-V versus V-I color-magnitude diagram, they are approximately 2 mag subluminous compared to normal disk dwarfs with solar-like metallicities, occupying a region also populated by O-rich subdwarfs with -1.5 < [M/H] < -1.0. The kinematics indicate that they are members of the Galactic spheroid population The subluminosity of all three stars is due to an as yet unknown combination of (undoubtedly low) metallicity, possibly enhanced helium abundance, and unusual line blanketing in the bandpasses considered. The properties of the stars are compared with models for the production of dwarf carbon stars.