Be Star/Pulsar Systems
The transient X-ray sources discovered in the 1970's were initial separated into two class based on spectra: Soft X-ray Transients (now thought to be black-hole binaries) and Hard X-ray Transients (HXTs). In 1975, pulsations were observed from the HXTs A1118-616 and A0535+26. In the same year a model was proposed to explain transient outbursts as due to a compact object in an eccentric binary system, accreting from the stellar wind of its companion. It was noticed in 1976 that many of the optical counterparts of X-ray transients were B emission line stars (Be stars). These observations and theory were brought together in 1978, with the determination of a wide eccentric orbit for the Be star/pulsar binary 4U 0115+63.