Bobby Darin (born Walden Robert Cassotto; May 14, 1936 - December 20, 1973) was an American singer, songwriter, multi-instrumentalist, and actor in film and television. He performed jazz, pop, rock and roll, folk, swing, and country music. He started his career as a songwriter for Connie Francis.

Born Walden Robert Cassotto in the East Harlem neighborhood of New York City, Bobby Darin was reared by his maternal grandmother, whom he believed was his mother. Darin's birth mother, Vanina Juliette "Nina" Cassotto (born November 30, 1917), became pregnant with him in the summer of 1935 when she was 17. Presumably because of the scandalous nature of out-of-wedlock pregnancies in that era, Nina and her mother hatched a plan to pass her baby off as Nina's younger brother. Years later, when Nina finally told Darin the truth about his upbringing, she refused to reveal the identity of his biological father, and kept the secret to her death in 1983. Darin's maternal grandfather, Saverio Antonio "Big Sam Curly" Cassotto (born January 26, 1882), was of Italian descent and a would-be mobster who died in prison from pneumonia a year before Darin's birth. His maternal grandmother, Vivian Fern Walden (also born in 1882), who called herself Polly, was of English ancestry and a vaudeville singer. From his birth, Darin always believed Nina to be his older sister and Polly his mother. But in 1968, when he was 32, Nina told Darin the truth, reportedly devastating him.  By the time he was a teenager, Darin could play several instruments, including piano, drums, and guitar. He later added harmonica and xylophone.  Darin moved to the Bronx early in his life (with a rented summer home in Staten Island) and graduated from the prestigious Bronx High School of Science. In later years he attributed his arrogance to his experiences there, where he was surrounded by brighter students who teased him. He then enrolled at Hunter College and soon gravitated to the drama department. After only two semesters, he dropped out to pursue an acting career.

Using a quote from the above article, answer the following question: Who was his father?
she refused to reveal the identity of his biological father, and kept the secret to her death in 1983.