  The X-ray transient MAXI J1631-479 went into outburst on 2018 December 21 and
remained active for about seven months. Owing to various constraints it was
monitored by NICER only during the decay phase of the outburst for about four
months. The NICER observations were primarily in the soft state with a brief
excursion to the hard intermediate state. While the soft state spectrum was
dominated by thermal disk emission, the hard intermediate state spectrum had
maximum contribution from the thermal Comptonization. Almost all
intermediate-state power spectra had a Type-C low frequency quasi-periodic
oscillation (within 4 - 10 Hz), often accompanied by a harmonic component. The
frequency of these oscillations increased and the fractional rms decreased with
inner-disk temperature suggesting a geometric origin. One observation in the
middle of the outburst during the hard intermediate state had two
non-harmonically related peaks. While one of them was definitely a Type-C QPO,
the identification of the other one is uncertain. The rms spectra during the
intermediate state had a hard shape from above 1 keV. Below 1 keV the shape
could not be constrained in most cases, while only a few observations showed a
rise in amplitude.
