Andrew Byrd, The Indo-European Syllable (2015), page 117: the reader should note that monosyllabic lengthening did not occur in PGmc. *hʷát "what" (< PIE *kʷód; not PGmc. ˟hʷṓt) or *át "at" (< PIE *ád; not PGmc. ˟ṓt), two words of similar shape (Ringe 2006:98).
^ De Vaan, Michiel (2008) Etymological Dictionary of Latin and the other Italic Languages (Leiden Indo-European Etymological Dictionary Series; 7)[2], Leiden, Boston: Brill, →ISBN
^ Ačaṙean, Hračʻeay (1979) “ց”, in Hayerēn armatakan baṙaran [Armenian Etymological Dictionary] (in Armenian), 2nd edition, a reprint of the original 1926–1935 seven-volume edition, volume IV, Yerevan: University Press, page 446a