The two are members of the Office to Foster Unity (Daftare Tahkim Vahdat), Iran's largest pro-reform student group.
In an interview with Radio Farda, Hedayat's mother, Mahbubeh Javadi, said a group of security officers entered her house in Tehran early on July 13, saying they had a search order.
According to Javadi, the officers arrested Hedayat without any explanation. They also confiscated a number of compact discs and books.
Both Hedayat and Hashemi have yet to be formally charged, Radio Farda reports.
The two activists spent a month in Evin prison last year after taking part in a peaceful demonstration.
Radio Farda reports that the latest arrests come amid renewed pressure by the authorities on Iran's student movement.
At least 13 student activists were arrested earlier this month in the capital, Tehran, and in Mashhad.