Cal. Probate Code - PROB § 21700. Section 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.