To cite HeckmanEM in publications use:

Lachos VH, Prates MO, Dey DK (2021). “Heckman selection-t model: Parameter estimation via the EM-algorithm.” Journal of Multivariate Analysis, 184, 104737.

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 = {Journal of Multivariate Analysis},
    volume = {184},
    pages = {104737},
    year = {2021},
  }