bsddb3 database file, what are the __db.001, __db.002, __db.003 files for?

[Claudio Grondi]

I have just started to play around with the bsddb3 module interfacing 
the Berkeley Database.

Beside the intended database file
   databaseFile.bdb
I see in same directory also the
   __db.001
   __db.002
   __db.003
files where
   __db.003 is ten times as larger as the databaseFile.bdb
and
   __db.001 has the same size as the databaseFile.bdb .

What are these files for and what could be the reason they occur?

If I delete them, the access to the database in databaseFile.bdb
still works as expected.

Any hints toward enlightenment?

Is there any _good_ documentation of the bsddb3 module around beside 
this provided with this module itself, where it is not necessary e.g. to 
guess, that C integer value of zero (0) is represented in Python by the 
value None returned in case of success by db.open() ?

Claudio