[XeTeX] Macro for kerning adjustment

[Sven Siegmund]

Hello,

I have to write phonetic transcriptions which involve nasalized glide
[w]. So I have to have a sequence of e.g. [auwa] where all the letters
have a tilde for nasalization above them.

There are single unicode characters for a, e, i, o, u, y with tilde,
so I use them. However, a w with tilde above is afaik not defined in
unicode 5.0. The solution is to write "w" followed by the U+0303
COMBINING TILDE. I use Gentium font and with Gentium this renders as a
w with a tilde which is far too right above. So I adjusted the kerning
manually by "\kern-0.085em{}" My sequence of nasalized [auwa] has now
the source code of:

[ãũw\kern-0.085em{}̃\kern+0.085em{}ã]
(you can hardly see the combining tilde because it combines with "}")
So there is \kern-0.085em between w and combining tilde and a corresponding
̃\kern+0.085em{} between the combining tilde and ã to make them again
properly kerned.

This works ok for me, but I don't want to do this manually. I will
have to write the nasalized w a plenty of times and would like to know
how can I set xelatex up in a way it automatically will kern the
sequence w + combining tilde like this (and therafter switch to normal
kerning again)

S.