Vaivasvata performed a fire sacrifice to obtain a son (putrakamyeshti). He got a daughter, Ila. Sage Vasishtha, at the request of Vaivasvata, transformed her into a son, Sudyumna. He later became Ila again, met Budha, Mercury, their son was Pururavas. He fell in love with Urvashi . Their son was Ayu.
Read about Pururavas and Urvashi here