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The period derivative is the rate at which the period of some cycle is changing as time passes, a period such as an orbital period or an object's rotation period. It is often indicated by a P with a dot over it, often pronounced p-dot:
. P (meaning dP/dt)
Examples with astrophysical interest include period derivatives of the rotation of pulsars (which gradually slow down; their rotation rate is sometimes monitored, e.g., for pulsar timing arrays), and period derivatives of the orbits around compact objects, such as that of the Hulse-Taylor Binary which demonstrated the effects of gravitational waves.