2003 Domain with 2008 Additional Controller

Hi,

I have been asked a question with which am not sure how to answer.

We have a Windows 2003 domain, with one domain controller. I have been asked can we add an additional domain controller but can it be Server 2008 R2?

I would think it could be possible if the forest and domain levels are kept at 2003, the current situation is some redundancy needs to be added to the domain, but budget is preventing all servers from being upgraded.

Thanks
Karl