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XSS J12270− 4859 is an X-ray binary associated with the Fermi Large Area Telescope gamma-ray source 1FGL J1227. 9− 4852. In 2012 December, this source underwent a transition where the X-ray and optical luminosity dropped and the spectral signatures of an accretion disk disappeared. We report the discovery of a 1.69 millisecond pulsar (MSP), PSR J1227− 4853, at a dispersion measure of 43.4 pc cm− 3 associated with this source, using the Giant Metrewave Radio Telescope (GMRT) at 607 MHz. This demonstrates that, post-transition, the system hosts an active radio MSP. This is the third system after PSR J1023+ 0038 and PSR J1824− 2452I showing evidence of state switching between radio MSP and low-mass X-ray binary states. We report timing observations of PSR J1227− 4853 with the GMRT and Parkes, which give a precise determination of the rotational and orbital parameters of the system …