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Discovery of a hot ultramassive rapidly rotating DBA White Dwarf

We report the discovery of a nearby massive white dwarf with He-H atmosphere. The white dwarf is located at a distance of $74.5\pm0.9$ pc. Its radius, mass, effective temperature, H/He ratio and age are $R=2500\pm100$ km, $M=1.33\pm0.01$ $\rm M_{\odot}$, $T_{\mathrm{eff}}=31200\pm 1200$ K, ${\mathrm{H/He}}\sim0.1$ and $330\pm40$ Myr, respectively. The observed spectrum is redshifted by $V_{\mathrm{r}}=+240\pm15$ km s$^{-1},$ which is mostly attributed to the gravitational redshift. The white dwarf shows a regular stable photometric variability with amplitude $Δg\approx0.06^{\mathrm{m}}$ and period $P=353.456$ s suggesting rapid rotation. This massive, hot and rapidly rotating white dwarf is likely to originate from the merging of close binary white dwarf system that avoided explosion in a thermonuclear type Ia supernova at the Carboniferous Period of the Earth history.