Preprocessed C++ source. Need information about a program.

Hi I'm involved in a project to renew an old managament program.
I've copied the whole folder to examine the language programming and database used.
The program folder has .AT Files (Preprocessed C++ source file) .I files .D files and .DAT Files.

At the beginnig I thought .D files were the database maybe progress database (but the folder hasnt other database files as schemas or similar..), but now I'm not sure and maybe are compiled objects (I don't know how to open it).
Every week the people on this office backup the whole folder, that's the reason I though the database is inside and maybe are the .D files....
By the other hand, .DAT files stores some data, but seems more like temp files or dumped files...

What do you know? How can I know what db type is used by this program? How can I localize the database? Is it possible reverse engineering with some of these files?

Thanks in advance!!