Ulupi was the daughter of a Naga King. She had a son named Iraban. His father was Arjuna. What was the marital status of Ulupi during her marriage with Arjuna? Was she a virgin? or a widow?
Arjuna spent one night with Ulupi and begot a son named Iravat. Ulupi's husband was killed by Garuda a long time ago and she was living in her father's palace. Arjuna gave Ulupi the status of a wife even though there was no official marriage between them. Here is the citation from Bhishma Parva:
This handsome and valiant son of Arjuna, named Iravat, was begotten upon the daughter of the king of the Nagas by the intelligent Partha. Her husband having been slain by Garuda, she became helpless, and of cheerless soul. Childless as she was, she was bestowed (upon Arjuna) by the high-souled Airavat. Partha accepted her for wife, coming to him as she did under the influence of desire. It was thus that that son of Arjuna was begotten upon the wife of another.
After the kurukshetra war, Ulupi lived in Hastinapura palace until Pandavas left for heaven.
Ulupi was not a widow as she says
This verse is in Adiparva, chapter 213, verse number 20
Here is the English translation from The Mahabharata of Krishna Dwaipayana Vyasa translated by Kisari Mohan Ganguly (Book I: Adi Parva, Arjuna Vanavasa Parva Section CCXVI)
"Hearing these words of Arjuna, Ulupi answered, 'There is a Naga of the name of Kauravya, born in the line of Airavata. I am, O prince, the daughter of that Kauravya, and my name is Ulupi. O tiger among men, beholding thee descend into the stream to perform thy ablutions, I was deprived of reason by the god of desire. O sinless one, I am still unmarried. Afflicted as I am by the god of desire on account of thee, O thou of Kuru's race, gratify me today by giving thyself up to me.'