He is also a co-discoverer of the three periodic comets 121P/Shoemaker-Holt, 127P/Holt–Olmstead, and 128P/Shoemaker-Holt.
並且他是週期彗星121P/Shoemaker-Holt、127P/Holt-Olmstead 和128P/Shoemaker-Holt的共同發現者。