What is the vow (nadhr) that necessitates fasting: is it one month or two months, in the case where a person vowed to fast for one month, then vowed to fast for two months for the same thing?

If one vows to fast for two months, then he must fast for two months. If his intention was to add to the first vowed month, then he must fast for two additional months, making a total of three months, because a vow is based on intention. He is obliged to fast for at least two months, and the majority of jurists do not stipulate continuous fasting unless it is specified by intention.