[XeTeX] combining overline and other unicode features

[Bernd]

Hello,
I've been playing around with the transliteration of Arabic using 
Unicode Characters. I found out that combining characters for dots and 
bars exist but I have no idea how to apply them. They copy fine from 
other texts but how do I type these myself? Is there a combination I 
have to enter on my keyboard as you do with accents and graves on 
vowels? Or do I need to enter the unicode number plus the letter?

Thank you, Bernd.