  General relativity predicts that short orbital period binaries emit
significant gravitational radiation, and the upcoming Laser Interferometer
Space Antenna (LISA) is expected to detect tens of thousands of such systems;
however, few have been identified, and only one is eclipsing--the double white
dwarf binary SDSS J065133.338+284423.37, which has an orbital period of 12.75
minutes. Here, we report the discovery of an eclipsing double white dwarf
binary system with an orbital period of only 6.91 minutes, ZTF
J153932.16+502738.8. This system has an orbital period close to half that of
SDSS J065133.338+284423.37 and an orbit so compact that the entire binary could
fit within the diameter of the planet Saturn. The system exhibits a deep
eclipse, and a double-lined spectroscopic nature. We observe rapid orbital
decay, consistent with that expected from general relativity. ZTF
J153932.16+502738.8 is a significant source of gravitational radiation close to
the peak of LISA's sensitivity, and should be detected within the first week of
LISA observations.
