A pulsar in orbit with another star. The existence of a companion star is revealed by a cyclic change in the pulse period as the two stars orbit each other. Over 250 binary pulsars were known by 2017, with orbital periods from an hour and a half to several years, and pulsation periods from 1.4  ms (millisecond pulsars) to over 2 s. The first known binary pulsar, PSR 1913+16, was discovered in 1974. It consists of a pulsar that pulses 17 times a second, in a highly eccentric orbit of period 7.75 hours around a second neutron star from which pulses are not observed. Each star is of 1.4 solar masses, close to the Chandrasekhar limit, and the orbital period is gradually shortening due to loss of energy through gravitational radiation. Other notable binary pulsars include PSR 1957+20, sometimes called the Black Widow Pulsar, in which intense radiation from the pulsar is evaporating its small companion star. The first example in which both members are detectable pulsars (i.e. a double pulsar), PSR J0737–3039AB, was discovered in 2003; in this system a pulsar with a period of 23 ms is orbited every 2.4 hours by a pulsar with a period of 2.8 s. A number of binary pulsars are now known to be recycled pulsars which have been spun up to high rotational speeds by accretion of gas from the companion.