transparency in vray

[giraffe]

hi I am very new in vray.
I couldnt figure out which parameter defines transparency of an object in vray.
I examined the material settings, read the help files and even checked some of the water elements posted to the forums but I couldnot find out which setting makes a material transparent or not.
please help.

[egz]

refraction color

white = 100% transparent

[giraffe]

thanx