# An 8.8 minute orbital period eclipsing detached double white dwarf binary

### Abstract

We report the discovery of ZTF J2243+5242, an eclipsing double white dwarf binary with an orbital period of just $8.8$ minutes, the second known eclipsing binary with an orbital period less than ten minutes. The system likely consists of two low-mass white dwarfs, and will merge in approximately 400,000 years to form either an isolated hot subdwarf or an R Coronae Borealis star. Like its $6.91, rm min$ counterpart, ZTF J1539+5027, ZTF J2243+5242 will be among the strongest gravitational wave sources detectable by the space-based gravitational-wave detector The Laser Space Interferometer Antenna (LISA) because its gravitational-wave frequency falls near the peak of LISA’s sensitivity. Based on its estimated distance of $d=2120^{+131}_{-115},rm kpc$, LISA should detect the source within its first few months of operation, and should achieve a signal-to-noise ratio of $87pm5$ after four years. We find component masses of $MA= 0.349^{+0.093}{-0.074},M_odot$ and $MB=0.384^{+0.114}{-0.074},M_odot$, radii of $RA=0.0308^{+0.0026}{-0.0025},R_odot$ and $RB = 0.0291^{+0.0032}{-0.0024},R_odot$, and effective temperatures of $TA=22200^{+1800}{-1600},rm K$ and $TB=16200^{+1200}{-1000},rm K$. We determined all of these properties, and the distance to this system, using only photometric measurements, demonstrating a feasible way to estimate parameters for the large population of optically faint ($r>21 , m_{rm AB}$) gravitational-wave sources which the Vera Rubin Observatory (VRO) and LISA should identify.

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