 The currencies of Sweden (the Swedish krona), the three largest countries which joined the European Union on 1 May 2004 (the Polish złoty, the Czech koruna, and the Hungarian forint), the two countries which joined on 1 January 2007 (the Bulgarian lev, and the Romanian leu) as well Croatia, which joined the EU on 1 July 2013 (the Croatian kuna), are required to join in accordance with the terms of the applicable treaties of accession.Sweden has voted in a referendum to stay out of the mechanism, despite being expected to join by the ECB, since Sweden has no opt-out like Denmark or the UK.EU members are required to join the ERM by the Maastricht convergence criteria.