Não informado. Observations of Variable Stars. Kapteyn Astronomical Laboratory.
Chicago-viite (17. p.)Não informado. Observations of Variable Stars. Groningen, Holanda: Kapteyn Astronomical Laboratory.
MLA-viite (8. p.)Não informado. Observations of Variable Stars. Kapteyn Astronomical Laboratory.
Varoitus: Nämä viitteet eivät aina ole täysin luotettavia.