NICER observation list: Target ID 664001

Ultracompact white dwarf binaries are the most compact binary systems known and can have orbital periods shorter than 5 minutes. Two such systems are of special interest as pulsating, soft X-ray sources for unique, quantitative studies of their orbital evolution. Both HM Cnc and V407 Vul show soft X-ray modulations at their orbital periods of 5.4 and 9.5 minutes, respectively. Timing the arrival of these pulses provides a powerful probe of orbital evolution in these very close binaries, where the interplay of torques associated with gravitational radiation and accretion set the evolution of their orbits. We propose NICER observations to further enable the long-term monitoring of their orbital periods.