The peculiar M giant HD 154791. Optical counterpart of the X-ray source 4U1700+24

HD 154791 is the only optical counterpart of an hard X-ray source (4U1700+24) 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 derive a possible scenario involving a wide binary system, where an old, slowly rotating, neutron star is weekly interacting with its late optical companion. We foresee that this system could evolve, in a short time, into a normal LMXB. Based on data collected at the Loiano Observatory