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While his estranged wifey Jennifer Lopez has been running around town with her young boyfriend Casper Smart, Marc Anthony has been hanging solo at a Beverly Hills hotel.

The father of two, looking quite thin, is reportedly furious over JLo's new relationship ... especially the fact that Smart has been bonding with his twins, Max and Emme.

But don't feel too bad for Mr. Anthony. The Latin crooner is dating young Mexican model Adriana Ontiveros, according to Life & Style.

All's fair in love and war.