Helena Petrovna Gan (Russian: Елена Петровна Ган, also Hélène, 12 August [O.S. 31 July] 1831, Yekaterinoslav, Yekaterinoslav, Russian Empire (today Dnipropetrovsk, Ukraine) – May 8, 1891, London), better known as Helena Blavatsky (Russian: Елена Блаватская) or Madame Blavatsky, born Helena von Hahn, was a founder of Theosophy and the Theosophical Society.