HD 154791 is the only optical counterpart of an hard X-ray source (4U1700+2
4) with an apparently normal M giant spectrum. We present here new high S/N
spectroscopic observations of this interesting and poorly studied object,
pointing out a number of peculiarities never noticed before; therefore we d
erive a possible scenario involving a wide binary system, where an old, slo
wly rotating, neutron star is weekly interacting with its late optical comp
anion. We foresee that this system could evolve, in a short time, into a no
rmal LMXB.