A Second Stellar Color Locus: a Bridge from White Dwarfs to M stars
  We report the discovery of a locus of stars in the SDSS g-r vs. 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. 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. (1995). 
V. Smolcic, Z. Ivezic, G.R. Knapp, R.H. Lupton, K. Pavlovski, S.   Ilijic, D. Schlegel, J.A. Smith, P.M. McGehee, N.M. Silve...