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In this work, the temporal and spectral properties of the poorly studiedX-ray pulsar Swift J1808.4$-$1754 were investigated in the 0.8-79 keV energyrange based on the data from the NuSTAR and Swift observatories collectedduring the 2014 outburst. Strong pulsations with a period of $909.73\pm0.03$ swere detected in the source light curve, with the pulsed fraction demonstratinga nonmonotonic dependence on the energy with a local minimum around 17-22 keV.Phase lags in one of the pulse profile components, reaching the maximal valueapproximately at the same energy, were discovered. The pulse phase-averagedspectrum of the source has a power-law shape with an exponential cutoff at highenergies, which is typical of X-ray pulsars. Pulse phase-resolved spectroscopyrevealed the presence of a pulse phase-transient cyclotron absorption line at$\sim$21 keV, allowing us to estimate the neutron star magnetic field of$2.4\times10^{12}$ G. This makes Swift J1808.4$-$1754 a member of very smallfamily of X-ray pulsars with a pulse-phase-transient cyclotron line in a narrowphase range. The data from the Nordic Optical Telescope allowed us to study theproperties of the IR companion in the system and to conclude that most probablyit is a Be-type star located at a distance of 5-8 kpc.

Discovery of a pulse-phase-transient cyclotron line in the X-ray pulsar Swift J1808.4-1754 and identification of an optical companion