A HZ Herculis star is a low-mass X-ray pulsar that is also an optically variable binary in which mass transfer is taking place. Such stars consist of a dwarf primary of spectral type B to F, and a neutron star that is an X-ray pulsar and possibly also an optical pulsar. The X-ray emission flips periodically between a high state and a low star, with peak optical luminosity corresponding to the high state.