  We present long slit spectral (375-725 nm) observations of the proposed
identification of the transient 66s X-ray pulsar Cep X-4=GS2138+56. Spectra
show features typical of Be/X-ray binaries. Superimposed on a weak emission
from the IC 1396 nebula, strong H$\alpha$ (4.5 nm EW) and H$\beta$ (0.3 nm EW)
lines are seen in emission with the other Balmer lines in absorption.
Significant interstellar absorption features are also detected, including a
strong Na I doublet (589 nm) and diffuse bands at 443, 578 and 628 nm. From the
shape of the continuum as well as the lines present in the spectrum, a most
probable spectral type of B1-B2V is derived with a reddening of
E$_{B-V}$=1.3$\pm$0.1. The reddening value is corroborated by the measure of
interstellar absorption features, except for the sodium line which appears to
be in excess and may be partly from circumstellar origin. The optical
absorption is fully consistent with the column density derived from X-ray
spectra, therefore confirming the identification. Despite apparent spatial
coincidence, the source is located much further away than the local intervening
nebula IC1396. The best estimate of the distance is D=(3.8$\pm$0.6) kpc, which
places the source in the outer Perseus arm of the Galaxy. At this distance, the
X-ray quiescent luminosity is (3-6) 10$^{33}$ erg.s$^{-1}$, thus comparable to
typical Be/X-ray binary low states.
