Orbital period changes in the eclipsing pulsar binary PSR B1957+20

PSR B1957+20 is a 1.6 ms pulsar orbited by a ∼0.025 M⊙ companion which eclipses the pulsar’s radio signal during approximately ten percent of the 9.2 hour orbit. The eclipses are thought to be caused by an ionized ‘‘cometary’’ wind trailing the companion in its orbit and constantly infused with new matter. The companion has been optically identified—its luminosity is strongly modulated with the orbital period of the system, consistent with synchronous rotation and heating by irradiation from the pulsar. We thus appear to be watching a pulsar in the process of slowly evaporating its companion, the star which presumably spun it up to millisecond periods during an earlier low‐mass x‐ray binary phase in the system’s evolution.