Civil Code § 981

(a) Unless otherwise agreed, an original work of authorship not fixed in any tangible medium of expression and in the creation of which several persons are jointly concerned, is owned by them in equal proportion.

(b) Unless otherwise agreed, an invention or design in the production of which several persons are jointly concerned is owned by them as follows:

(1) If the invention or design is single, in equal proportions.

(2) If it is not single, in proportion to the contribution of each.