Namaste

Recently I saw a reference to some Upanishads called the "Tapani" Upanushads, such as the Radhika Tapani Upanishad and Gopala Tapani Upanishad and many others.

What are these? I never heard of them. Is a Tapani Upanishad some commentary or Appendix to an actual Upanishad but authored later by some initiated savant of a particular school?

Thank you in advance.

Om Namah Sivaya