OAO1657-415 : A `Missing Link' in High Mass X-ray Binaries?
  OAO1657-415 is only the seventh eclipsing X-ray pulsar known and therefore has the potential to yield only the seventh mass of a neutron star in an X-ray binary. Here we report photometric and spectroscopic observations of candidates for the optical counterpart to the system and identify a B6V or B5III star as a possible companion to the neutron star. We measure the observational parameters of the star and suggest reasons why OAO1657-415 may be unlike other high mass X-ray binaries. 
D.H. Maxwell (1), A.J. Norton (1) and P. Roche (2) ((1) The Open   University, (2) Leicester University)