Properties of sunspots in cycle 23 - I. Dependence of brightness on sunspot size and cycle phaseS. K. Mathew, V. Martínez Pillet, S. K. Solanki and N. A. KrivovaA&A, 465 1 (2007) 291-304DOI: https://doi.org/10.1051/0004-6361:20066356