org-mode, ledger, and separate ledger files

[Bill]

I'm running ledger 2.6.2 and org-mode 7.01.g on Debian Squeeze, and I'd 
like to use org-babel to integrate the two.  All the examples I've found 
show the actual ledger contained in the org-mode file itself, but I'd like 
to connect to an existing ledger file.  What is the magic incantation, 
assuming there is one?  Where do I find that documentation?

Thanks,

Bill