  The detection of coherent X-ray pulsations at ~314 Hz (3.2 ms) classifies
MAXI J1957+032 as a fast-rotating, accreting neutron star. We present the
temporal and spectral analysis performed using NICER observations collected
during the latest outburst of the source. Doppler modulation of the X-ray
pulsation revealed the ultra-compact nature of the binary system characterised
by an orbital period of ~1 hour and a projected semi-major axis of 14 lt-ms.
The neutron star binary mass function suggests a minimum donor mass of 1.7e-2
Msun, assuming a neutron star mass of 1.4 Msun and a binary inclination angle
lower than 60 degrees. This assumption is supported by the lack of eclipses or
dips in the X-ray light curve of the source. We characterised the 0.5-10 keV
energy spectrum of the source in outburst as the superposition of a relatively
cold black-body-like thermal emission compatible with the emission from the
neutron star surface and a Comptonisation component with photon index
consistent with a typical hard state. We did not find evidence for iron K-alpha
lines or reflection components.
