Kernrechtsfrage
Whether the public-law appeal was admissible against the interlocutory joinder decision
Extrahierter Entscheid
The decision was immediately challengeable under the case law on joinder of third parties, because waiting until the final judgment would defeat procedural economy.
Extrahierte Begründung
Admission of a third party into the lawsuit has effects akin to a joinder of causes; if review were deferred, an inadmissible joinder would force a new trial. An immediate remedy is therefore justified, unless the appeal is manifestly dilatory.