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This is a useful and timely paper. As the authors note, there have been little study of overlay journals and the contribution is novel. The identification and classification of 34 overlay journals is very useful, and the paper appears to paint an accurate picture of the state of overlay journals. The main omission is a lack of discussion of SSRN research paper series (https://www.ssrn.com/index.cfm/en/rps/), some being available via subscription.

Some specific points:

• The introduction is well-written and contains appropriate citations.
• It is helpful to include a definition of an overlay journal.
• The search for journals has some limitations. Alternative terms for preprints such as working paper or white paper could have been included, and searching for peer-reviewed preprints may have brought up additional venues.
• The use of English terms for searching is likely to have led to a bias in the results towards English journals, which should be noted.
• The paper would benefit from a discussion of long-term achiving (CLOCKSS/LOCKSS/Portico), are they relying on the repositories to do this for them and is this a risk for overlay journals?
• I respect the authors’ choices, however I think one could make a strong argument for the inclusion of journals based on the F1000 platform since papers that are not approved by peer reviewers stay on the platform so it operates as both journal and repository. This is a grey area that highlights the evolution of preprints and peer review, and the blurring of boundaries between preprints and traditional journals.