B737-800 CREW AND A B727 HAD LESS THAN LEGAL SEPARATION IN ATL CLASS B AIRSPACE.

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TKOF CLRNC: FLY 070 DEG HDG AT MARKER; CLRED FOR TKOF RWY 8R ATL. AFTER ROTATION; WE WERE IMC AND INSTRUCTED TO CONTACT DEP 125.7. INITIAL HDG WAS 092 DEGS UNTIL MARKER. CLBING THROUGH 1000 FT AGL; WHILE SWITCHING CONFIGN AND FREQS; WE GOT A TA. WE BROKE OUT OF CLOUDS AROUND 1100 FT AGL AND WE WERE TURNING TO ASSIGNED HDG OF 070 DEGS. COPLT SAW A B727 TURNING N INTO US JUST AS WE BROKE OUT OF CLOUDS. COPLT CALLED OUT THE TFC TO THE CAPT AND TOLD THE CAPT TO CONTINUE TURING L AND STOP CLBING. WE STARTED TO GET AN RA; 'MONITOR VERT SPD.' DEP CTLR CALLED US TO SEE IF WE WERE ON FREQ YET. COPLT TOLD HIM YES AND WE HAD THE B727 IN SIGHT. DEP CTLR TOLD US WE COULD TURN N IMMEDIATELY IF NECESSARY. I TOLD HIM WE WERE ALREADY IN THE TURN. THE CAPT STOPPED HIS CLB AT 2100 FT MSL; AND ASKED COPLT TO KEEP THE B727 IN SIGHT. THE B727 SEEMED TO BE CORRECTING HIS TURN AND BEGAN TURNING BACK TOWARDS S (BELLY UP TO US). WE STAYED BELOW AND UNDERNEATH HIM. WE MISSED HIM BY ONLY A FEW HUNDRED FT THE RA WAS SLOW TO GIVE PROPER INPUTS. LUCKILY WE HAD JUST BROKEN OUT OF THE WX AND SAW HIM VISUALLY AND LEVELED OFF IMMEDIATELY. WE BEGAN OUR AVOIDANCE MANEUVER BEFORE THE RA; BUT AFTER RECEIVING THE TA. AFTER CLR OF CONFLICT; WE ASKED DEP TO VERIFY ASSIGNED HDG AND ALT. THE CAPT ASKED; 'WHAT WAS THAT?' THE CTLR RESPONDED; 'WE ARE INVESTIGATING IT.' WE WERE INSTRUCTED 360 DEG HDG AND 10000 FT MSL; 250 KTS.