Please cite R.oo/R.methodsS3 as

Bengtsson H (2003). “The R.oo package - Object-Oriented Programming with References Using Standard R Code.” In Hornik K, Leisch F, Zeileis A (eds.), Proceedings of the 3rd International Workshop on Distributed Statistical Computing (DSC 2003). https://www.r-project.org/conferences/DSC-2003/Proceedings/Bengtsson.pdf.

Corresponding BibTeX entry:

  @InProceedings{,
    author = {Henrik Bengtsson},
    title = {The {R.oo} package - Object-Oriented Programming with
      References Using Standard {R} Code},
    booktitle = {Proceedings of the 3rd International Workshop on
      Distributed Statistical Computing (DSC 2003)},
    year = {2003},
    editor = {Kurt Hornik and Friedrich Leisch and Achim Zeileis},
    address = {Vienna, Austria},
    month = {March},
    issn = {1609-395X},
    url =
      {https://www.r-project.org/conferences/DSC-2003/Proceedings/Bengtsson.pdf},
    howpublished =
      {https://www.r-project.org/conferences/DSC-2003/Proceedings/},
  }