A Second Stellar Color Locus: a Bridge from White Dwarfs to M stars

We report the discovery of a locus of binary stars in the Sloan Digital Sky Survey (SDSS) g-r versus u-g color-color diagram that connects the colors of white dwarfs and M dwarfs. While its contrast with respect to the main stellar locus is only ~1 : 2300, this previously unrecognized feature includes 863 stars from the SDSS Data Release 1 (DR1). The position and shape of the feature are in good agreement with predictions of a simple binary star model that consists of a white dwarf and an M dwarf, with the components' luminosity ratio controlling the position along this binary system locus. SDSS DR1 spectra for 47 of these objects strongly support this model. The absolute magnitude-color distribution inferred for the white dwarf component is in good agreement with the models of Bergeron et al.