  We investigated the optical, X-ray, and gamma-ray variability of the pulsar
SAX J2103.5+4545. Our timing and spectral analyses of the X-ray and gamma-ray
emissions from the source using RXTE and INTEGRAL data show that the shape of
its spectrum in the energy range 3 -- 100 keV is virtually independent of its
intensity and the orbital phase. Based on XMM-Newton data, we accurately (5
arcsec) localized the object and determined the optical counterpart in the
binary. We placed upper limits on the variability of the latter in the R band
and the H-alpha line on time scales of the orbital and pulse periods,
respectively.
