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There is a striking and unexplained dearth of brown dwarf companions in closeorbits (< 3AU) around stars more massive than the Sun, in stark contrast to thefrequency of stellar and planetary companions. Although rare and relativelyshort-lived, these systems leave detectable evolutionary end points in the formof white dwarf - brown dwarf binaries and these remnants can offer uniqueinsights into the births and deaths of their parent systems. We present thediscovery of a close (orbital separation ~ 0.006 AU) substellar companion to amassive white dwarf member of the Praesepe star cluster. Using the cluster ageand the mass of the white dwarf we constrain the mass of the white dwarfprogenitor star to lie in the range 3.5 - 3.7 Msun (B9). The high mass of thewhite dwarf means the substellar companion must have been engulfed by the Bstar's envelope while it was on the late asymptotic giant branch (AGB). Hence,the initial separation of the system was ~2 AU, with common envelope evolutionreducing the separation to its current value. The initial and final orbitalseparations allow us to constrain the combination of the common envelopeefficiency (alpha) and binding energy parameters (lambda) for the AGB star toalpha lambda ~3. We examine the various formation scenarios and conclude thatthe substellar object was most likely to have been captured by the white dwarfprogenitor early in the life of the cluster, rather than forming in situ.

WD0837+185:the formation and evolution of an extreme mass ratio white dwarf-brown dwarf binary in Praesepe