 those standards which have not changed since the early 20th century, are S Monocerotis (O7 V) and 10 Lacertae (O9 V).The Morgan–Keenan–Kellerman (MKK) "Yerkes" atlas from 1943 listed O-type standards between O5 and O9, but only split luminosity classes for the O9s.The two MKK O9 V standards were Iota Orionis and 10 Lacertae.