SYS and other inbuilt accounts

[Ripin Natani]

Hi,
    I created a database instance and added a dba user. I logged in the SQL studio 
using the dba account I created. I can see 3 more user accounts besides the one I 
created: 
1. sysdba
2. Domain
3. SYS
Can some one tell me what is the default password for this SYS account ? Also, are 
there any other inbuilt accounts that get created when creating an instance ?
Thanks,
-Ripin.