To cite varstan in publications use:

  Alonzo, I. and Cruz, C. (2020). varstan: An R Package for Bayesian
  time series models with Stan. arxiv preprint
  doi:https://arxiv.org/abs/2005.10361

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{varstan}: An {R} Package for {Bayesian} time series models with {Stan}},
    author = {Izhar Asael Alonzo and Cristian Cruz},
    journal = {ARXIV Preprint},
    year = {2020},
    encoding = {UTF-8},
  }