HM Cancri is a candidate ultracompact binary white dwarf with an apparent orbital period of only 5.4 minutes , as suggested by X-ray and optical light-curve modulations on that period , and by the absence of longer-period variability .
In this Letter we present Keck-I spectroscopy which shows clear modulation of the helium emission lines in both radial velocity and amplitude on the 5.4-minute period and no other .
The data strongly suggest that the binary is emitting He i 4471 from the irradiated face of the cooler , less massive star , and He ii 4686 from a ring around the more massive star .
From their relative radial velocities , we measure a mass ratio q = 0.50 \pm 0.13 .
We conclude that the observed 5.4-minute period almost certainly represents the orbital period of an interacting binary white dwarf .
We thus confirm that HM Cnc is the shortest-period binary star known : a unique test for stellar evolution theory , and one of the strongest known sources of gravitational waves for the Laser Interferometer Space Antenna ( LISA ) .