Gadhi (King)

King Gadhi’s wife gave birth to Kaushika, who became sage Vishvamitra. Gadhi was not the biological father.

Gadhi’s daughter Satyavati was married with sage Richika. She asked for a mixture for herself und her mother to have children.

Richika mixed a drink for the queen to give birth to a ruler (Kshatriya) and one for Satyavati, to give birth to a sage (Brahmana). The mother exchanged the fessels with the mixture. The queen gave birth to a sage, Kaushika, and Satyavati to a ruler, Jamadagni.

Read the story here
http://hindumythologyforgennext.blogspot.com/2012/04/sage-vishwamitra-part-1.html