Fade in/fade out effect

ok guys i need help
in my movie i have button and when i rollover
over the button , its start contiunosly fade in/fade out effect until i rollout the mouse its have to stop , now i have the script but its works just when i rollover and rollout the mouse and then again i need to rollover again , i searched for this script in forum but didnt find the code for contiunosly effect…
here is my script …this script goes in my actionscript layer :

b1.onRollOver = function() {
_alpha -= 7;

_alpha = 100;

help me please …

Uhh… shouldn’t you just tell it to behave as a movie clip? I didn’t read this too thoroughly but that appears to be the problem…

the effect i want to achieve is a button fadein and fade out
when u roll over and stay with cursor…