He is also a co-discoverer of the three periodic comets 121P/Shoemaker-Holt, 127P/Holt–Olmstead, and 128P/Shoemaker-Holt.
Este co-descoperitor al cometelor periodice 121P/Shoemaker-Holt, 127P/Holt-Olmstead și 128P/Shoemaker-Holt.