  Although supernovae is a well-known endpoint of an accreting white dwarf,
alternative theoretical possibilities has been discussing broadly, such as the
accretion-induced collapse (AIC) event as the endpoint of oxygen-neon (ONe)
white dwarfs, either accreting up to or merging to excess the Chandrasekhar
limit (the maximum mass of a stable white dwarf). AIC is an important channel
to form neutron stars, especially for those unusual systems, which are hardly
produced by core-collapse supernovae. However, the observational evidences for
this theoretical predicted event and its progenitor are all very limited. In
all of the known progenitors, white dwarfs increase in mass by accretion. Here,
we report the discovery of an intriguing binary system Lan 11, consisted of a
stripped core-helium-burning hot subdwarf and an unseen compact object of 1.08
to 1.35 $M_{\odot}$. Our binary population synthesis calculations, along with
the absence of detection from the deep radio observations of the
Five-hundred-meter Aperture Spherical Radio Telescope, strongly suggest that
the latter is an ONe white dwarf. The total mass of this binary is 1.67 to 1.92
$M_{\odot}$}, significantly excessing the Chandrasekhar limit. The reproduction
of its evolutionary history indicates that the unique system has undergone two
phases of common envelope ejections, implying a born nature of this massive ONe
white dwarf rather than an accretion growth from its companion. These results,
together with short orbital period of this binary (3.65 hours), suggest that
this system will merge in 500-540 Myr, largely triggering an AIC event,
although the possibility of type Ia supernova cannot be fully ruled out. This
finding greatly provides valuable constraints on our understanding of stellar
endpoints, whatever leading to an AIC or a supernova.
