To cite HeckmanEM in publications use:

Lachos VH, Prates MO, Dey DK (2020). “Heckman selection-t model: parameter estimation via the EM-algorithm.” arXiv preprint arXiv:2006.08036. https://arxiv.org/abs/2006.08036.

Corresponding BibTeX entry:

  @Article{,
    title = {Heckman selection-t model: parameter estimation via the
      EM-algorithm},
    author = {V{\'i}ctor Hugo Lachos and Marcos Oliveira Prates and
      Dipak Kumar Dey},
    journal = {arXiv preprint arXiv:2006.08036},
    year = {2020},
    url = {https://arxiv.org/abs/2006.08036},
  }