  SWIFT J1626.6-5156 is an X-ray pulsar that was discovered in December 2005
during an X-ray outburst. Although the X-ray data suggest that the system is a
high-mass X-ray binary, very little information exists on the nature of the
optical counterpart. We have analysed all RXTE observations since its
discovery, archived optical spectroscopic and photometric data and obtained for
the first time near-IR spectra. The K-band spectrum shows HeI 20581 A and HI
21660 A (Brackett-gamma) in emission, which confine the spectral type of the
companion to be earlier than B2.5. The H-band spectrum exhibits the HI Br-18-11
recombination series in emission. The most prominent feature of the optical
band spectrum is the strong emission of the Balmer line Halpha. The 4000-5000 A
spectrum contains HeII and numerous HeI ines in absorption, indicating an early
B-type star. The source shows three consecutive stages characterised by
different types of variability in the X-ray band: a smooth decay after the peak
of a large outburst, large-amplitude flaring variability (reminiscent of type I
oytbursts) and quiescence. We observed that the spectrum becomes softer as the
flux decreases and that this is a common characteristic of the X-ray emission
for all observing epochs. An emission line feature at ~6.5 keV is also always
present. The X-ray/optical/IR continuum and spectral features are typical of an
accreting X-ray pulsar with an early-type donor. The long-term X-ray
variability is also characteristic of hard X-ray transients. We conclude that
SWIFT J1626.6-5156 is a Be/X-ray binary with a B0Ve companion located at a
distance of ~10 kpc.
