SQL & PL/SQL

hello all

plz,
can we put a referenced field as a primary key?

i have this type :

c+reate type tservicep as object (+
+ refnump ref tprofesseurp,+
+ refnomc ref tclassep,+
+ nbh integer+
+ );+

and i want to create the table of this type
when do it with :

CREATE TABLE SERVICEP OF TSERVICEP (PRIMARY KEY (refnumP,refnomC));

I get this error :

ERROR at line 1:
ORA-02329: column of datatype REF cannot be unique or a primary key



thanks