English to German Bus/Financial Government / Politics

Es geht um die Beitrittskandidaten und die Frage, ob diese den Euro akzeptieren werden. Opt-out kommt in zwei Sätzen vor. Eine griffige Lösung ist mir bisher nicht eingefallen, nur ein Nebensatzwurm. Wer kann helfen:

In theory, the question is not if, but when, these countries will join the euro: unlike Britain and Denmark, which are in the EU but not the single currency, they have no Maastricht treaty "opt-out".
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And should the EU's new members be reluctant to join, they cannot be forced to. Sweden, an EU member with no opt-out, has stayed out, though it will hold a referendum this September.