no joint return shall be made if the husband and wife have different taxable years; except that if such taxable years begin on the same day and end on different days because of the death of either or both, then the joint return may be made with respect to the taxable year of each. The above exception shall not apply if the surviving spouse remarries before the close of his taxable year, nor if the taxable year of either spouse is a fractional part of a year under section 443(a)(1);