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Convention (XI) relative to certain Restrictions with regard to the Exercise of the Right of Capture in Naval War. The Hague, 18 October 1907.
Chapter III : Regulations regarding the crews of enemy merchant ships captured by a belligerent - Art. 5.
Art. 5. When an enemy merchant ship is captured by a belligerent, such of its crew as are nationals of a neutral State are not made prisoners of war.
The same rule applies in the case of the captain and officers likewise nationals of a neutral State, if they promise formally in writing not to serve on an enemy ship while the war lasts.