I am very grateful to M. Gross for explaining to me the right normalization of the holomorphic volume form and several other key issues. Also I have considerably benefited from conversations with R. Bryant, C. Haase, M. Kontsevich, D. Morrison and M. Stern. Finally, I would like to thank IHES for its hospitality and financial support during the summers of 2001 and 2002 where a significant part of the work has been done.