St. Thomas Aquinas, the Angelic Doctor and Prince of Theologians, on head coverings for women:
Hence Augustine says (Ep. ccxlv.) in a letter to Possidius: "I do not wish you to be hasty in forbidding the wearing of gold or costly attire except in the case of those who being neither married nor wishful to marry, should think how they may please God: whereas the others think on the things of the world, either husbands how they may please their wives, or wives how they may please their husbands, except that it is unbecoming for women though married to uncover their hair, since the Apostle commands them to cover the head." Yet in this case some might be excused from sin, when they do this not through vanity but on account of some contrary custom: although such a custom is not to be commended.
Summa Theologica, Second Part of the Second Part, Q. 169, Article 2