Vajrasuci (Siddhacarya Ghoshapada) 10

10

viśvāmitraṃ21 ca caṇḍālī22 vasiṣṭhaṃ23 caiva urvaśī
na teṣāṃ brāhmaṇī mātā lokācārācca brāhmaṇāḥ

and a caṇḍāla woman to Viśvāmitra, and Urvaśī indeed to Vasiṣṭha.
Their mothers were not brāhmaṇa women, yet they are brāhmaṇas by worldly custom.