Are you listed on the Deed to the property as well? If so, then you are required to sign the Mortgage as well because all title-holders must sign to acknowledge the Lender's interest in the property and be made aware that the Lender has the right to foreclose if payments are not being made.
Your wife is most likely the only name listed on the Note, which makes her solely responsibile for paying back the loan. Obviously, you want to make sure those payments are being made so the Lender does not foreclose. Just because you signed the Mortgage does not necessarily mean that you are on the Note.
Were you present at the closing? You may want to look back at your copy package to see if your name was on the original mortgage, TIL, right to cancel and a few other documents at the closing.
Do you mean that you are both listed as borrowers on the mortgage, or just that you both signed the mortgage? If you and your wife are both listed on the deed to the property, even if your wife was the only borrower for the loan, the bank usually would have required you to sign the mortgage to show your consent to the lien on the property. That by itself would not make you a co-borrower on the loan. The bank would not be able to add you to the mortgage without your consent. You should have an attorney review your mortgage, and the other documents from your closing, to confirm what your obligations are under the mortgage.
Did you sign it? It is not correct to state that they "added you to" a document which you signed.
If you're a property owner, you generally are required to sign the mortgage. However, this does not necessarily obligate you to make payments on the note.
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