The prototype of intermittent pulsators comma HETE J1900.1-2455

HETE J1900.12455 is a highly peculiar accreting millisecond pulsar, as itspulses disappeared soon after the discovery of the source. Therefore, it can beviewed as a prototype of a system in between pulsating and non pulsating lowmass Xray binaries. We propose to observe it for the first time withXMMNewton, to discriminate between models explaining the pulse disappearance.The proposed observation will firmly constrain the inner disc radius thanks tothe modelling of the broad iron line that the source shows. Simultaneously, apulse search at an unprecedented large sensitivity will be carried over. Theresults obtained will be used to shed light on the reason that causes themajority of low mass Xray binaries hosting a neutron star not to show pulsations. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]