heptad vs septet

heptad

noun

  • A sequence of seven bases. 

  • Synonym of septet: a group of seven things. 

septet

noun

  • A set of seven lines. 

  • A group of seven musicians. 

  • A composition for a group of seven musicians. 

How often have the words heptad and septet occurred in a corpus of books? (source: Google Ngram Viewer )