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4XMM~J182531.5$-$144036: A new persistent Be/X-ray binary found within the \emph{XMM-Newton} serendipitous survey

We aim to investigate the nature of time-variable X-ray sources detected in the {\it XMM-Newton} serendipitous survey. The X-ray light curves of objects in the {\it XMM-Newton} serendipitous survey were searched for variability and coincident serendipitous sources observed by {\it Chandra} were also investigated. Subsequent infrared spectroscopy of the counterparts to the X-ray objects that were identified using UKIDSS was carried out using {\it ISAAC} on the VLT. We found that the object 4XMM~J182531.5--144036 detected in the {\it XMM-Newton} serendipitous survey in April 2008 was also detected by {\it Chandra} as CXOU~J182531.4--144036 in July 2004. Both observations reveal a hard X-ray source displaying a coherent X-ray pulsation at a period of 781~s. The source position is coincident with a $K=14$ mag infrared object whose spectrum exhibits strong HeI and Br$γ$ emission lines and an infrared excess above that of early B-type dwarf or giant stars. We conclude that 4XMM~J182531.5--144036 is a Be/X-ray binary pulsar exhibiting persistent X-ray emission and is likely in a long period, low eccentricity orbit, similar to X Per.