RYCHLEWSKA, U., BUDĚŠÍNSKÝ, Miloš, SZCZEPANSKA, B., BLOSZYK, E., HOLUB, Miroslav. Two Crystalline Forms of Sesquiterpene Lactone Helenalin - X-Ray and NMR Study. Collection of Czechoslovak Chemical Communications. 1995, 60(2), 276-299. ISSN 0010-0765.