[XeTeX] Combining consonant with elongated r vowel (ऋ) in Devanagari using Mac

[Ravi Parimi]

Hello ,

I am trying to join the consonant त with the vowel ऋ on my Mac running
10.5.6 but cannot find the right combination of keystrokes to get the
desired conjunct form. Combining त and the shorter vowel ृ works as
expected resulting in तृ

Can anyone on this list show me the right sequence of keys to obtain
this?  I'm using Devanagari QWERTY keyboard for inputting characters.

Thanks,
Ravi