^Bonatto, C.; Bica, E.; Santos, J. F. C., Jr. Spatial dependence of 2MASS luminosity and mass functions in the old open cluster NGC 188. Astronomy and Astrophysics. April 2005, 433 (3): 917–929. Bibcode:2005A&A...433..917B. doi:10.1051/0004-6361:20041113.