 They have all worked extensively with harpsichordist Gustav Leonhardt.On 2 February 2007 he was awarded the degree of Doctor Honoris Causa by the Catholic University of Leuven (Katholieke Universiteit Leuven, Belgium).Based on the results of research by Joshua Rifkin and Andrew Parrott on the authentic performance of Bach's masses, cantatas and passions, Sigiswald Kuijken limits the vocal part to a solo vocal quartet (which Bach himself sometimes extended to eight singers) instead of a choir.