Constraining the ortho-to-para ratio of H2 with anomalous H2CO absorption N. Troscompt, A. Faure, S. Maret, C. Ceccarelli, P. Hily-Blant and L. Wiesenfeld A&A, 506 3 (2009) 1243-1247 Published online: 03 September 2009 DOI: 10.1051/0004-6361/200912770