  Studies of deuterated isotopologues of complex organic molecules can provide
important constraints on their origin in regions of star formation. In
particular, the abundances of deuterated species are very sensitive to the
physical conditions in the environment where they form. Due to the low
temperatures in regions of star formation, these isotopologues are enhanced to
significant levels, making detections of multiply-deuterated species possible.
However, for complex organic species, only the multiply-deuterated variants of
methanol and methyl cyanide have been reported so far. The aim of this paper is
to initiate the characterisation of multiply-deuterated variants of complex
organic species with the first detection of doubly-deuterated methyl formate,
CHD2OCHO. We use ALMA observations from the Protostellar Interferometric Line
Survey (PILS) of the protostellar binary IRAS 16293-2422, in the spectral range
of 329.1 GHz to 362.9 GHz. We report the first detection of doubly-deuterated
methyl formate CHD2OCHO in the ISM. The D/H ratio of CHD2OCHO is found to be
2-3 times higher than the D/H ratio of CH2DOCHO for both sources, similar to
the results for formaldehyde from the same dataset. The observations are
compared to a gas-grain chemical network coupled to a dynamical physical model,
tracing the evolution of a molecular cloud until the end of the Class 0
protostellar stage. The overall D/H ratio enhancements found in the
observations are of the same order of magnitude as the predictions from the
model for the early stages of Class 0 protostars. However, the higher D/H ratio
of CHD2OCHO compared to the D/H ratio of CH2DOCHO is still not predicted by the
model. This suggests that a mechanism is enhancing the D/H ratio of singly- and
doubly-deuterated methyl formate that is not in the model, e.g. mechanisms for
H-D substitutions.
