Pub 590 and Pub 17

Generally, you can take a deduction for the contributions that you are allowed to
   make to your IRA. However, if you or your spouse were covered by an
   employer retirement plan at any time during 1997 and you made contributions,
   your allowable IRA deduction may be less than your allowable contributions. Your
   deduction may be reduced or eliminated, depending on the amount of your income
   and your filing status, as discussed later under Deduction Limits. These limits do
   not affect your allowable contributions (see Nondeductible Contributions, later).

Marital status. Generally, for 1997, you were considered covered by an
   employer retirement plan if your spouse was covered by one. To determine
   whether you were considered covered for the tax year because of your spouse's
   coverage, you had to wait until the last day of the year. This is because your filing
   status (whether you are considered married or single) for the year depends on your
   marital status on the last day of the tax year. 

Married filing separate exception. If you did not live with your spouse at any
   time during the year and you file a separate return, your filing status is considered,
   for this purpose, as single.