Milli Vanilli's Fab Morvan Got Secretly Married More Than 20 Years Ag

Fab Morvan was thought to be a longtime bachelor, but his recent divorce smashed that reputation -- now we know when exactly he got hitched ... and it was a while ago.

According to new legal docs, obtained by TMZ, the Milli Vanilli singer's estranged wife Kim Marlowe -- who filed for divorce earlier this week -- lists their marriage date as Dec. 3, 1998 ... while going on to point out they separated just earlier this year ... on Jan. 2.

She cites irreconcilable differences as the official reason they're going their separate ways -- and she's asking for spousal support from Fab. She also wants to make it so FM can't ask for any alimony on his end. The couple apparently didn't have any kids together.

In terms of how she wants to divvy up their shared marital assets ... Marlowe says all that should be figured out in the discovery process as their divorce plays out in court.

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Pretty standard stuff all in all, but again -- the noteworthy thing is that Fab here seems to have tied the knot secretly more than 2 decades ago ... and nobody knew about it!

Now, it's worth noting ... Marlowe is claiming they separated just a couple weeks ago -- which is shocking, 'cause Fab has been very publicly living his life with his current partner, Tessa Van Der Steen, and their 4 children ... with whom he apparently lives in Amsterdam.