In any collaboration with external parties, the arm's length principle means that you have to make sure that you have the freedom of research to conduct the project with the contributions the external party wishes to include. There must be no doubt that your research and communication are independent of any decisive influence from external parties, and that you as a researcher always have the final word. Transparency and open declaration in the process are crucial: write what you and the external party have done every time.
Yes, and this must be agreed in the contract in advance. The external party may only suggest clarifications or explanations of the content and conclusions of the draft, and comments on any misunderstandings in relation managing the basis made available for the project by the external party. Comments from external party must be written and kept on the project file. You should note your own thoughts on the external comments, and then store your memo on the project file as well. Make sure that you maintain your research integrity when you deal with the comments.
The general rule for authorship is described in the AU policy for research integrity (see link at the end of this document). To qualify as an author, the contribution to the product must contain as a minimum:
As a general rule, there should be no joint authorship between AU researchers and external parties on products published by AU. It must be agreed in advance whether and for what reason external parties can be co-authors or contributors. If there are any contributions in the form of data from the external party, this must be clearly stated in the product as well as how this data has been included. The product must also state if and how external parties have contributed in any other way to the project. If an external party is involved as the author of a chapter, the chapter should clearly show what the external party has authored.
An appendix with the comments from the external party, and the researcher's opinion on these comments, should be kept on the project file. It is important to ensure your academic integrity. Correspondence should be filed, and the first part of the product should describe how the external party has contributed.