De bono viduitatis
This work is not mentioned in the Retractations, probably because it is a letter, and as such it is reckoned by Possidius, cap. 7. It is also marked as St. Augustine's by its references to his other works, De Bono Conjugali, etc. cap. 15. Ep. to Proba, cap. 23. The date is marked by the recent consecration of Demetrias, which was in 413. The admonition for which he is thanked by Juliana, Epistle 188, may be that against Pelagianism.
An objection has been raised from its disagreement with the fourth Council of Carthage, an. 398. can. 104, which excommunicates widows who marry again after consecration, and pronounces them guilty of adultery, whereas in cap. 10 and 11, the opinion that such marriages are no marriages, and that they ought to return to continence, is refuted. The two, however, are not wholly irreconcileable, as there may be a guilt similar to that of adultery incurred, and it may be visited with a censure in the form of excommunication, and yet the marriage may remain valid. The 16th Canon of Chalcedon imposes such a penalty, with power to the Bishop to relax it.— Abridged from the Benedictine Edition.
Augustine the Bishop, servant of Christ, and of the servants of Christ, unto the religious handmaiden of God, Juliana, in the Lord of lords health.
Source: On the Good of Widowhood (New Advent)