Optical studies of companions to millisecond pulsars

Optical observations of the companions of pulsars can help determine the properties of the binaries, as well as those of their components, and give clues to the preceding evolution. In this review, we first describe the different classes of binary pulsars, and present a table with a summary what is known about their optical counterparts. Next, we focus on the class of pulsars that have low-mass, helium-core white dwarf companions. We discuss attempts to determine the masses of both components using optical spectroscopy, and compare the pulsar spin-down ages with cooling ages of the white dwarfs. We confirm that for a given age, the lowest-mass white dwarfs are much hotter than the more massive ones, consistent with recent evolutionary models, although with one glaring exception. We discuss the case of PSR B0820+02, where the cooling age indicates a braking index less than 3, and conclude by describing how cooling ages can be used to test formation scenarios for PSR J1911-5958A, a pulsar binary in the outskirts of NGC 6752.