请问这里的which指代的是什么成分

Ukraine has been the subject of several of our By Invitation columns—The Economist’s equivalent of op|eds—of which we publish several each week and which have become increasingly popular in recent months.

请老师看一下，这里是否非限制性从句，这里的which指代的是前面什么成分呢？

which 指columns,定语从句假如前面没逗号，尽可能看作限定性定从，尽管有时非限定性从句也有不需要逗号的状况。了解定语从句的修饰关系，比是不是限定性更要紧。