Problem with OJB and Inheritance hierarcy

Hi,

I am using OJB for the first time. I have three tables, say T1, T2 and
T3. These tables are the ER representation of three java classes C1, C2
and C3. From the object orientation point of view C1 is the super class
of the other two.
When I had to map the classes with the tables I've used the "table per
subclass" method (map every subclass to a distinct table).

The problem is :
when I try to delete an Object of class C2 I expect that OJB delete the
C2 object and even the referenced C1 object. But this does not happens.

How could I achieve this behaviour?

Thanks
Vincent