Two pulsars in orbit around each other, as distinct from a binary in which only one member is a pulsar (see binary pulsar). The first, and so far only, example of a double pulsar, J0737–3039AB, was discovered in 2003 in the constellation Puppis. Each of the pulsars has a mass about 1.3 times that of the Sun, and their orbital period is 2.4 hours. The theory of general relativity predicts that such a binary should lose energy through the emission of gravitational waves, which makes the double pulsar an excellent natural laboratory for testing relativity. As deduced from the change in orbital period, the separation of the pulsars is reducing by about 7 mm per day, in exact agreement with theory. It is expected that the stars will eventually merge in approximately 85 million years. In addition, the Shapiro delay in the travel time of the pulse from the more distant pulsar as it passes through the curved space around the nearer one is exactly as predicted.