NMAC BTWN A PART 135 EMS FLT AND AN EXPERIMENTAL ACFT OUTSIDE THE 30 NM VEIL OF ZZZ; US. DEP CTLR HAD ALERTED EMS CREW OF THE TFC.

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WE WERE IN LEVEL FLT AT 4000 FT ABOUT 35 NM W OF ARPT HDG NW. WE WERE OPERATING ON AN IFR CLRNC DIRECT AND WERE BEING HELD AT 4000 FT UNTIL CLR OF THE APCH CORRIDOR. THE WX CONDITIONS WERE BROKEN CLOUDS AT 3500 FT WITH TOPS AROUND 5000 FT. ATC TOLD US OF TFC AT 9 - 10 O'CLOCK; 3 MI. WE REPLIED WITH 'LOOKING; BUT WE ARE IN AND OUT IMC.' LESS THAN 1 MIN LATER; WE BROKE OUT OF A CLOUD TO FIND A SINGLE ENG ACFT AT 12 - 1 O'CLOCK LESS THAN 1/2 MI XING FROM R TO L. THE FO IMMEDIATELY DEVIATED UP AND TO THE R TO AVOID THIS ACFT. WE CLBED APPROX 500 FT; CLRED THE ACFT; AND RETURNED TO OUR ASSIGNED ALT. I ATTEMPTED TO INFORM ATC OF OUR DEV; BUT THERE WAS NO OPPORTUNITY TO CONTACT ATC UNTIL AFTER WE RETURNED TO 4000 FT DUE TO CONGESTION ON THE FREQ. ATC CALLED US AT THEIR FIRST OPPORTUNITY AND SIMPLY INSTRUCTED US TO CONTACT CTR. NO OTHER COMMENTS WERE MADE ABOUT THIS EVENT BY US OR BY ATC. THE CONTRIBUTING FACTORS WERE THAT WE WERE OPERATING OUTSIDE THE MODE C XPONDER VEIL AND THIS ACFT APPEARED TO BE AN EXPERIMENTAL (POSSIBLY WITHOUT A XPONDER); THAT HE WAS OPERATING VFR IN LESS THAN VFR CONDITIONS; AND ATC'S WORKLOAD WAS HIGH.