  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 $d=2120^{+131}_{-115}\,\rm pc$,
LISA should detect the source within its first few months of operation, and
should achieve a signal-to-noise ratio of $87\pm5$ after four years. We find
component masses of $M_A= 0.349^{+0.093}_{-0.074}\,M_\odot$ and
$M_B=0.384^{+0.114}_{-0.074}\,M_\odot$, radii of
$R_A=0.0308^{+0.0026}_{-0.0025}\,R_\odot$ and $R_B =
0.0291^{+0.0032}_{-0.0024}\,R_\odot$, and effective temperatures of
$T_A=22200^{+1800}_{-1600}\,\rm K$ and $T_B=16200^{+1200}_{-1000}\,\rm K$. 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.
