  We present follow-up studies of three ultracompact hot subdwarf binaries.
Using data from the Zwicky Transient Facility, we find orbital periods of 33.6,
37.3, and 36.9 minutes for ZTF 1946+3203, ZTF 0640+1738, and ZTF 0643+0318
respectively. The light curves show ellipsoidal variability of the hot subdwarf
star with potential eclipses of an accretion disc. Phase-resolved spectroscopic
observations with Keck were used to measure a radial velocity curve and
atmospheric parameters of the hot subdwarf stars. ZTF J0643 shows evidence of
accretion disc emission lines in the average spectrum. Combining light curve
and spectroscopic fits will allow us to measure precise system properties such
as masses, to determine the evolutionary history and future evolution of the
system.
