heptad vs sextet

heptad

noun
  • A sequence of seven bases. 

  • Synonym of septet: a group of seven things. 

sextet

noun
  • A composition for six voices or instruments. 

  • Any group of six people or things. 

  • A group of six singers or instrumentalists. 

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