CLOSE PROX ACR-LGT GA-SMA NORTHEAST OF CVG AT NIGHT.

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WE WERE AT 6000' MSL CLRED (IFR) TO 4500' MSL BY CVG APCH CTL. JUST AS THE F/O (FLYING THE ACFT) STARTED DSNDING; I NOTICED TFC AT OUR 12 O'CLOCK POS; SLIGHTLY BELOW US (LIGHTS BLENDING IN WITH THE LIGHTS ON HORIZON) POS LIGHTS ONLY--NO LNDG LIGHT ON; MOVING VERY SLIGHTLY FROM OUR LEFT TO OUR RIGHT (MOVEMENT ALMOST UNDETECTED). I IMMEDIATELY BROUGHT IT TO THE ATTN OF THE PLTS. F/O IMMEDIATELY TURNED LEFT 30 DEGS AND PUSHED THE NOSE DOWN TO 8 DEGS. THE OTHER ACFT APPARENTLY SAW US AT THAT TIME BECAUSE HE ILLUMINATED HIS LNDG LIGHT AND TURNED TO HIS LEFT. THE LIGHT ACFT PASSED OFF OUR RIGHT WING SLIGHTLY ABOVE US. OUR LNDG LIGHTS HAD BEEN TURNED ON AT 18000'. HE DEFINITELY SHOULD HAVE SEEN US EARLIER. HE PASSED OF OUR RIGHT SIDE APPROX 200' ABOVE US AND 400' HORIZ TO OUR RIGHT. SEVERAL OFF OUR PAX SAW HIM ZIP BY THE RIGHT SIDE WINDOWS. ATC WAS QUERIED; PAINTING NO TFC. POINTS: PROBABLY THE SMA WAS CLBING TO 6500'. APPEARS HE WAS LEGAL VFR. WE ONLY SAW HIS POS LIGHTS AT THE LAST SAFE MOMENT. NO STROBES; NO LNDG LIGHT; WEAK RED BEACON. OUR 3-M CREW MOST LIKELY AVOIDED A MIDAIR. A 2-M CREW PROBABLY WOULD NOT HAVE; DUE TO WORKLOAD. OUR CREW HAS BEEN WELL RESTED AND ALERT. GOOD EMPHASIS ON ANTI JET LAG SCHEDULING. LIGHT ACFT TOOK NO EVASIVE ACTION UNTIL WE HAD. STROBES/LNDG LIGHTS ON HIS ACFT WOULD HAVE AVERTED THE SITUATION. TRANSPONDER WOULD PROBABLY HAVE AVERTED IT ALSO.