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If you are divorced, but your marriage lasted 10 years or longer, you can receive benefits on your ex-spouse's record (even if he or she has remarried) if:
If you remarry, you generally cannot collect benefits on your former spouse's record unless your later marriage ends (whether by death, divorce or annulment).
If your ex-spouse has not applied for retirement benefits, but can qualify for them, you can receive benefits on his or her record if you have been divorced for at least two years.
See here for details : http://www.ssa.gov/retire2/divspouse.htm