The nature of the unseen companion to WD0837+185

WD0837+185 is a radial velocity (RV) variable, white dwarf (WD) member of the Praesepe open star cluster. Using high precision RV measurements we estimate the binary period as 4.2 hr, with a velocity semi amplitude of ~11 km/s, but other periods are possible. Photometry obtained from UKIRT indicates that there is no near-IR excess (in J, H or K), making the secondary either a cool T (>T5) dwarf with mass ~30 MJup, the lowest mass companion to a WD known, or a massive (>1.2MSun) WD. In this latter case the system has a super-Chandrasekhar mass and is a candidate Type 1a Supernova progenitor and, will converge within a Hubble time. If the period is considerably longer (> 12hrs), the companion is certainly another WD, but will not converge within a Hubble time. We have applied for time to obtain high resolution radial velocity measurements to confirm our estimate of the period. We therefore request IRAC 3.6 and 4.5 micron observations of this system to determine whether the companion is a low mass T dwarf, or a more massive WD.