It was unclear on Monday who would succeed Faymann, with the government in theory having two more years to govern. Christian Kern, currently the head of the national railways company, and Gerhard Zeiler, former chief of national broadcaster ORF, have been touted as possible replacements. Vice Chancellor Reinhold Mitterlehner, who is from the other establishment party, the conservative Austrian People’s Party, will serve as interim chancellor. Under Austria’s Constitution, Mr. Mitterlehner could remain in office days, weeks or even months.As the same time, Michael Häupl, the veteran mayor of Vienna, will take over as the temporary leader of the Social Democrats. Among the challenges facing Austria are the integration of about 90,000 refugees and migrants who applied for asylum last year.