Optical, J and K light curves of XTE J1118+480 = KV UMa: the mass of the black hole and the spectrum of the non-stellar component

 Optical, J and K photometric observations of the KV UMa black hole X-ray nova in its quiescent state obtained in 2017–2018 are presented. A significant flickering within light curves was not detected, although the average brightness of the system faded by ≈0.1m over 350 d. Changes in the average brightness were not accompanied with an increase or decrease in the flickering. From the modelling of five light curves the inclination of the KV UMa orbit and the black hole mass were obtained: i = 74 ± 4°, $M_\mathrm{BH}=(7.06\rm {-}7.24)\, \mathrm{M}_{\odot }$, depending on the mass ratio used. The non-stellar component of the spectrum in the range λ = 6400–22 000 Å can be fitted by a power law Fλ ∼ λα, α ≈ −1.8. The accretion disc orientation angle changed from one epoch to another. The model with spots on the star was inadequate. Evolutionary calculations using the scenario machine code were performed for low-mass X-ray binaries, with a recently discovered anomalously rapid decrease of the orbital period taken into account. We show that the observed decrease can be consistent with the magnetic stellar wind of the optical companion, whose magnetic field was increased during the common-envelope stage. Several constraints on evolutionary scenario parameters were developed.