Optical studies of the companion to the “redback” pulsar J1908+2105

The binary millisecond radio- and gamma-ray pulsar J1908+2105 is likely a member of the “redback” class. Its possible companion has been discovered in the Gaia catalog. To confirm the companion and to obtain parameters of the system, we carried out time-series photometry of the source in the $B$, $V$, $R$, $I$ bands with the 2.1-m OAN-SPM telescope in Mexico. We found its brightness variability with the period of 3.5 h consistent with the pulsar orbital period thus confirming the source association with the pulsar. Amplitudes of the variations are 0.2–0.3$^m$, typical for "redbacks" . We also performed optical spectroscopy of the companion with the 10-m Gran Telescopio Canarias. The obtained spectra correspond to a star of spectral class K-M.