The orbital period and inclination of the neutron star X-ray transient MAXI J1807+132

The neutron star X-ray transient MAXI J1807+132 has undergone outbursts in 2017, 2019, and 2023. We conducted an R-band time series photometry campaign using the Isaac Newton Telescope during the 2022 quiescent state. We detected a periodic variation in the light curve, consistent with ellipsoidal modulation, which allowed us to determine an orbital period of Porb = 4.258 ± 0.008 hr. By modelling the light curve, we obtained a binary inclination of i = 72±5 \, ° and a mass ratio q = 0.24+0.19-0.14 ($68$ per cent confidence level). Furthermore, our analysis supports an early M-dwarf companion that contributes between 30 and 50 per cent to the total flux in the R-band. We extend the previously established absolute magnitude versus orbital period correlation for black hole X-ray transients to neutron star systems. We applied the correlation to MAXI J1807+132, estimating its distance as 6.3 ± 0.7 kpc and its height above the Galactic plane to be 1.6 ± 0.2 kpc.