ENV vs BASH_ENV

Hi all,
I cannot find out the difference between defining the two variables, but I 
discovered after a lot that having both set up in my crontab avoids a script 
to run correctly. For instance if I have in my crontab:

ENV=/home/luca/.profile
BASH_ENV=/home/luca/.profile

then a shell script seems not to execute unless its output is redirect to a 
file. So the line

00 10 * * * myBackup.sh

does not seem to run (or starts and ends immediatly), while

00 10 * * * myBackup.sh > /tmp/backup.log

works fine. If I run the backup script manually it works (it uses rsync). 
What's the magic around the above variables?

Thanks,
Luca