Bigamous marriage null and void from the beginning

Dear PAO,

My husband was previously married when I married him 13 years ago. He said that he was already separated for five years with his previous wife before he met me. Our marriage was also not successful because he was a womanizer. Lately, I found out that the first wife filed a case to have their marriage with my husband declared as null and void. I just want to ask if I can also have our marriage annulled. What ground could I use for my annulment? What happens to the petition which I will file in case the petition filed by the first wife is dismissed?