Hypergeometric monodromy groups of orthogonal type

A result of Levelt completely describes the monodromy representation associated to the "Clausen–Thomae hypergeometric functions". Using this and certain results of Beukers and Heckman, we show that if the real Zariski closure of the monodromy group is O(p,q) with p and q at least two, then there are many such monodromy groups which are arithmetic. The roof exploits the existence of many unipotent elements in the monodromy group.