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We aim to investigate the nature of time-variable X-ray sources detected inthe {\it XMM-Newton} serendipitous survey. The X-ray light curves of objects inthe {\it XMM-Newton} serendipitous survey were searched for variability andcoincident serendipitous sources observed by {\it Chandra} were alsoinvestigated. Subsequent infrared spectroscopy of the counterparts to the X-rayobjects that were identified using UKIDSS was carried out using {\it ISAAC} onthe VLT. We found that the object 4XMM~J182531.5--144036 detected in the {\itXMM-Newton} serendipitous survey in April 2008 was also detected by {\itChandra} as CXOU~J182531.4--144036 in July 2004. Both observations reveal ahard 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 whosespectrum exhibits strong HeI and Br$\gamma$ emission lines and an infraredexcess above that of early B-type dwarf or giant stars. We conclude that4XMM~J182531.5--144036 is a Be/X-ray binary pulsar exhibiting persistent X-rayemission and is likely in a long period, low eccentricity orbit, similar to XPer.

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