Cannot delete Yosemite partition

Hi,
I installed Yosemite on a 2nd partition and now I want to get rid of it. When I boot to Mavericks, Disk Utility does not allow me to modify any of the two partitions ( - button disabled). When I boot to Recovery 10.9, I can only delete my Mavericks partition but not the Yosemite one. The - button is disabled when I select the 2nd partition.

How can I remove it ?

cheers

 

Yes, see my post.

 

Code:

diskutil cs revert disk1

thanks!