The action for the recognition of natural children may be brought only during the lifetime of the presumed parents, except in the following cases:
(1) If the father or mother died during the minority of the child, in which case the latter may file the action before the expiration of four years from the attainment of his majority;
(2) If after the death of the father or of the mother a document should appear of which nothing had been heard and in which either or both parents recognize the child.
In this case, the action must be commenced within four years from the finding of the document. (137a)