New variable stars in open clusters - I. Methods and results for 20 open clusters E. Paunzen, K. Zwintz, H. M. Maitzen, O. I. Pintado and M. Rode-Paunzen A&A, 418 1 (2004) 99-102 Published online: 02 April 2004 DOI: 10.1051/0004-6361:20035624