2005 California Probate Code Sections 21700 PART 7. CONTRACTS REGARDING TESTAMENTARY OR INTESTATE SUCCESSION ..

21700.  (a) A contract to make a will or devise or other instrument,
or not to revoke a will or devise or other instrument, or to die
intestate, if made after the effective date of this statute, can be
established only by one of the following:
   (1) Provisions of a will or other instrument stating the material
provisions of the contract.
   (2) An expressed reference in a will or other instrument to a
contract and extrinsic evidence proving the terms of the contract.
   (3) A writing signed by the decedent evidencing the contract.
   (4) Clear and convincing evidence of an agreement between the
decedent and the claimant or a promise by the decedent to the
claimant that is enforceable in equity.
   (5) Clear and convincing evidence of an agreement between the
decedent and another person for the benefit of the claimant or a
promise by the decedent to another person for the benefit of the
claimant that is enforceable in equity.
   (b) The execution of a joint will or mutual wills does not create
a presumption of a contract not to revoke the will or wills.
   (c) A contract to make a will or devise or other instrument, or
not to revoke a will or devise or other instrument, or to die
intestate, if made prior to the effective date of this section, shall
be construed under the law applicable to the contract prior to the
effective date of this section.