A DC9 FLC FINDS THEMSELVES 15 NM S OF COURSE WHEN NAVING OFF THE 306 DEG RADIAL OF ULW VOR; NY.

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AIRWAY DEV S OF COURSE BY APPROX 15 MI. WE WERE TRACKING OUTBOUND ON THE ULW 306 DEGS RADIAL AT APPROX 66 MI ON OUR WAY TO KOOPER INTXN (ULW 306 DEGS 96 KTS). FO WAS TUNED TO ULW AND CAPT HAD JUST TUNED TO NEXT FIX; YQO; WITH THE INTENT OF GETTING DIRECT INSTEAD OF INTERCEPTING THE YQO 097 DEG RADIAL TO YQO. WE WERE RECEIVING YQO; AND IT LOOKED AS THOUGH THE 097 DEG RADIAL WAS STARTING TO BECOME ACTIVE. AT THAT POINT; THE CABIN CREW CALLED FORWARD AND ASKED TO ENTER THE COCKPIT. DURING THE PROCESS; WHICH TOOK APPROX 3-5 MINS; THE FO WAS FLYING AND OBSERVED THAT THE RADIAL ON THE CAPT'S INST APPEARED TO BE ACTIVE. THE FO STARTED AN INTERCEPT WITH THE RADIAL AND SWITCHED TO YQO. AFTER THE CREW MEMBER LEFT THE COCKPIT AND WE HAD BOTH SECURED THE DOOR; ABOUT A MIN HAD PASSED WHEN ATC CALLED AND TOLD US WE HAD GONE 15 MI S OF COURSE. AT FIRST GLANCE; WE THOUGHT OUR INSTS WERE IN ERROR; BECAUSE WE WERE SHOWING OURSELVES WHERE WE THOUGHT WE WERE SUPPOSED TO BE. ATC THEN REFERRED TO ULW 306 DEG RADIAL; AND WE THEN RECHKED OUR POS; AND IT APPEARED AS THOUGH THERE WAS A DIFFERENCE BTWN THE CAPT AND FO'S INSTS; WHICH MAY HAVE BEEN PART OF THE PROB. WE DID WRITE UP THE PROB WITH THE FO VOR IN THE ACFT MAINT LOG. WE ALSO TALKED ABOUT WHAT ELSE COULD HAVE PUT US OFF COURSE. WE FIGURED OUT THAT IN THE PROCESS OF THE DISTR OF THE CREW MEMBER ENTERING THE COCKPIT; AND THE FO'S OBSERVATION OF WHAT HE THOUGHT WAS THE YQO 097 DEG RADIAL STARTING TO CTR UP; THAT WE MAY HAVE STARTED THE INTERCEPT TOO EARLY. DUE TO THE INTERRUPTION OF THE CALL FROM THE CABIN; I HAD NOT MENTIONED TO THE FO THAT I HAD JUST SWITCHED TO THE OTHER FREQ IN ANTICIPATION OF ASKING FOR THE DIRECT RTEING. I; AS CAPT; SHOULD HAVE MADE THE FO AWARE OF MY INTENT AT THE TIME OF THE SWITCHING TO YQO; AND THERE MAY NOT HAVE BEEN A PROB. ATC GAVE US VECTORS BACK ON COURSE AND THE REST OF THE TRIP WAS UNEVENTFUL.