SIMBAD references

We present a spectroscopic sample of 747 detached close binary systems from the Sloan Digital Sky Survey (SDSS) Fourth Data Release. The majority of these binaries consist of a white dwarf primary and a low-mass secondary (typically M dwarf) companion. We have determined the temperature and gravity for 496 of the white dwarf primaries and the spectral type and magnetic activity properties for 661 of the low-mass secondaries. We have estimated the distances for each of the white dwarf-main-sequence star binaries and use white dwarf evolutionary grids to establish the age of each binary system from the white dwarf cooling times. With respect to a spectroscopically identified sample of ∼8000 isolated M dwarf stars in the SDSS, the M dwarf secondaries show enhanced activity with a higher active fraction at a given spectral type. The white dwarf temperatures and gravities are similar to the distribution of ∼1900 DA white dwarfs from the SDSS. The ages of the binaries in this study range from ∼0.5 Myr to nearly 3 Gyr (average age ∼0.20 Gyr).