A lifetime ago — OK, it’s only been 46 years — Ryan O’Neal and starred as students at Harvard and Radcliffe in the impossibly romantic movie, “Love Story.’’ (McGraw was nominated for an Academy Award for her role in the film, which was written by Erich Segal based on his book of the same name.) Monday, the pair returned to the Cambridge campus to talk about “Love Letters,’’ the A.R. Gurney play they’re performing this week at Citi Shubert Theatre. The couple — he’s 74 and she’s 76 — arrived in an MG convertible similar to the car O’Neal’s character drove in “Love Story.’’