Interesting Features in the Combined GALEX and Sloan Color Diagrams of   Solar-like Galactic Populations
  We report on intriguing photometric properties of Galactic stars observed in the GALEX satellite's far-UV and near-UV bandbasses as well as from the SDSS survey and the Kepler Input Catalog. First, the FUV-NUV color distribution of stars in the Kepler field consists of 2 distinct peaks. Second, curiously, for stars with spectral types G or later the mean FUV-NUV color becomes much bluer, contrary to expectation. we have found in two samples of mid-F through K type stars that ~14-18% of them exhibit FUV-excesses relative to their NUV fluxes and spectral types. Nearly the same fractions are also discovered among members of the Kepler Eclipsing Binary Catalog and in the published list of Kepler Objects of Interest. These UV-excess ("UVe") colors are confirmed by the UV continuum slopes in GALEX spectra. The SDSS spectra of some UVe stars exhibit metallic line weakening especially in the blue. This suggests an enhanced contribution of UV flux relative to photospheric flux of a solar-type single star. We consider the possibility that the UV excesses originate from various types of hot stars, including binaries, and strong chromosphere stars that. Our model atmosphere-derived simulations of colors for binaries with main sequence pairs with a hot secondary demonstrate that the color loci conflict with the observed sequence. We are left with the active chromospheres explanation, whether in active binaries or young stars, as a still tentative explanation for the UVe stars. We also address the presence of an island of "UV red" stars in the NUV-g, g-i color diagram. The subpopulation comprising this island are mainly horizontal branch stars. These objects do not exhibit UV excesses and therefore have UV colors typical for their spectral types. This subpopulation appears "red" in the UV because their colors are not pulled to the blue by the UVe stars. 
Myron Smith, Luciana Bianchi and Bernard Shiao
astro-ph.SR