2006ApJ...638..446M

We provide the first age estimates for two recently discovered white dwarf-red subdwarf systems, LHS 193AB and LHS 300AB. These unusual systems provide a new opportunity for linking the reliable age estimates for the white dwarfs to the (measurable) metallicities of the red subdwarfs. We have obtained precise photometry in the V_JR_KCI_KCJH bands and spectroscopy covering from 6000 to 9000 Å for the two new systems, as well as for a comparison white dwarf-main-sequence red dwarf system, GJ 283 AB. Using model grids available in the literature, we estimate the cooling ages, as well as temperatures, surface gravities, masses, progenitor masses, and total lifetimes of the white dwarfs. The results indicate that the two new systems are probably ancient thick-disk objects with ages of at least 6-9 Gyr. We also conduct searches of red dwarf and white dwarf compendia from SDSS data, Lépine Shara Proper Motion (LSPM) catalog, and Chanamé & Gould for additional common proper motion white dwarf-red subdwarf systems. Only seven new candidate systems are found, which indicates the rarity of these systems.