  We present fully general relativistic (GR) simulations of binary white
dwarf-neutron star (WDNS) inspiral and merger. The initial binary is in a
circular orbit at the Roche critical separation. The goal is to determine the
ultimate fate of such systems. We focus on binaries whose total mass exceeds
the maximum mass (Mmax) a cold, degenerate EOS can support against
gravitational collapse. The time and length scales span many orders of
magnitude, making fully general relativistic hydrodynamic (GRHD) simulations
computationally prohibitive. For this reason, we model the WD as a
"pseudo-white dwarf" (pWD) as in our binary WDNS head-on collisions study
[PRD83:064002,2011]. Our GRHD simulations of a pWDNS system with a
0.98-solar-mass WD and a 1.4-solar-mass NS show that the merger remnant is a
spinning Thorne-Zytkow-like Object (TZlO) surrounded by a massive disk. The
final total rest mass exceeds Mmax, but the remnant does not collapse promptly.
To assess whether the object will ultimately collapse after cooling, we
introduce radiative thermal cooling. We first apply our cooling algorithm to
TZlOs formed in WDNS head-on collisions, and show that these objects collapse
and form black holes on the cooling time scale, as expected. However, when we
cool the spinning TZlO formed in the merger of a circular-orbit WDNS binary,
the remnant does not collapse, demonstrating that differential rotational
support is sufficient to prevent collapse. Given that the final total mass
exceeds Mmax, magnetic fields and/or viscosity may redistribute angular
momentum and ultimately lead to delayed collapse to a BH. We infer that the
merger of realistic massive WDNS binaries likely will lead to the formation of
spinning TZlOs that undergo delayed collapse.
