Teen star Mindy Cohn, who plays chubby Natalie on “The Facts of Life,” has been swept off her feet by the handsome younger brother of ex-teen idols David and Shaun Cassidy – and love has made her lose 25 pounds.

Sparks flew instantly when Mindy, 17, met Ryan Cassidy, also 17, at a friend’s birthday party. They’ve dated regularly since – and Mindy’s making sure she keeps Ryan’s eye on her by slimming down.

“Mindy’s gone from approximately 170 pounds in February to around 145 pounds now,” said a source close to her. “And it’s all inspired by her love for Ryan.”

And Mindy said to one friend: “I feel like Cinderella . . .it’s great. I love Ryan.”

Said the friend: “It’s like a fairy-tale romance for the two of them. Ryan has totally changed Mindy’s life.”

And a friend of Cassidy said: “Ryan is an extremely good-looking, well-liked guy. He could have his pick of any girl, but he spends all of his time with Mindy.

“Ryan told me, ‘Mindy’s the best thing for me right now – I love her, and I love being with her. Mindy’s totally unaffected and genuine.’”

Friends say Mindy had been depressed before meeting Ryan because she didn’t have a boyfriend – and MAD magazine had called her “Tubby” in a cartoon parody.

But Ryan’s changed all that. “He’s turned a troubled 17-year-old girl into a happy-go-lucky teenager in love,” said Mindy’s friend.

Recalling the couple’s first meeting, another pal said:

“At the birthday party, sparks between them flew right away. Mindy can have an engaging personality. She can be a wonderful, interesting person to talk to.”

“Ryan and Mindy gravitated toward each other and spent the rest of the day talking and laughing.”

Mindy and Ryan date constantly, and showed up together looking like two teens in love at an NBC party in Los Angeles. Said an eyewitness: “Mindy and Ryan were holding hands, had their arms around each other and were exchanging light and loving kisses. They made a great pair.”