Spectral analysis of 636 WD - M star binaries from the Sloan Digital Sky Survey (Data Release 6)

We present a catalog of 857 WD (WD)-M star binaries from the sixth data release of the Sloan Digital Sky Survey (SDSS), most of them known before. For 636 of them, we give a spectral analysis and derive the basic parameters of the stellar constituents and their distances from Earth. Our purpose is to test the modeled spectra applied in the fitting procedure on the one hand and to confine the respective evolutionary scenarios on the other hand. We use a χ2 minimization technique in order to decompose each combined spectrum and yield independent parameter estimates for the components. 41 of the stellar duets in our spectroscopic sample are optically resolved on the respective SDSS images. Despite various selection effects, the fraction of 6.4 % of WD-M star binaries with orbital separation around 500 AU and orbital perdiods of ∼ 104 yr is a criterion for evolutionary models of stellar binary systems. We find 20 out of 636 WDs being fitted as DOs, with 16 of them indicating TefWD around 40000K. This excess of cool DOs is most likely due to additional WDs in the DB-DO Teff range, for which no detailed fitting was done. Furthermore, we identify 70 very low-mass objects, meaning secondaries with masses smaller than about 0.1 Mo-dot, as candidate substellar companions. A trend of WD masses towards higher values in a binary constellation compared to those of field WDs is compatible with our results.