Credit: Kevin Mazur/VF1/

Who knew they were pals?

Natalie Portman and her 8-month son Aleph enjoyed the mild February temperatures in NYC on Saturday -- taking in the sunshine with none other than Macaulay Culkin.

Carrying Aleph (whose dad is Portman's fiance, Black Swan choreographer Benjamin Millepied) in a Baby Bjorn carrier, Portman, 30, and Culkin, 31, both wore jeans and sneakers and looked "super skinny," an observer tells Us Weekly. Culkin, in particular, says the witness, looked "emaciated."

Both former child stars, Portman and Culkin have been "friends for years," one source explains. As it happens, Culkin dated Portman's Black Swan costar Mila Kunis for eight years before calling it quits in late 2010.

Culkin sparked major concern among fans earlier this month when new photos emerged of the Home Alone star looking alarmingly gaunt. Countered his rep: "Macaulay Culkin is in perfectly good health."

Still, another source admitted that Culkin dealt with "emotional issues" for years.