Pulsation Modes of Cepheids
Some classical Cepheids pulsate simultaneously in two or even three modes.
Their lightcurves can be explained as a overposition of fundamental plus overtone modes.



The terminology you used is also found in this paper (where you have also other references):
"the discovery of many double-mode Cepheids (DMCs) pulsating in both the fundamental and first-overtone modes (F/10) and the first and second-overtone modes (10/20)"

Other reference: Classical cepheids