Nikki Sixx's Girlfriend Preps for Motley Crue Striptease

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Nikki Sixx is 53 ... his girlfriend is about to turn 27 ... and she plans to celebrate by stripping down to her undies for the rocker -- and 15,000 of his closest friends.

The birthday girl is Courtney Bingham ... and she's hot. Real hot. We have pics to prove it.

Anyway, we've learned ... she's planning a special burlesque performance to "Girls, Girls, Girls" ... and she plans to unveil the hotness on her birthday (9/5) during a Motley Crue show in Michigan.

FYI -- Court has been dating Sixx since 2010 -- after his rocky romance with Kat Von D bit the dust -- and they have been inseparable ever since.

Now we see why.