ABSTRACT Deuterium fractionation can constrain the physical and chemical conditions at early stage of brown dwarf formation. We present IRAM 30-m observations over a wide frequency range 213–279 GHz singly doubly deuterated species formaldehyde (HDCO D2CO) towards Class 0/I proto-brown dwarfs (proto-BDs). Multiple low-excitation HDCO D2CO transition lines with upper energy level ?40 K are detec...