  We report the first interstellar detection of c-C3D2. The doubly deuterated
cyclopropenylidene, a carbene, has been detected toward the starless cores TMC-
1C and L1544 using the IRAM 30m telescope. The J(Ka,Kc) = 3(0,3)-2(1,2),
3(1,3)-2(0,2), and 2(2,1)-1(1,0) transitions of this species have been observed
at 3 mm in both sources. The expected 1:2 intensity ratio has been found in the
3(0,3)-2(1,2) and 3(1,3)-2(0,2) lines, belonging to the para and ortho species
respectively. We also observed lines of the main species, c-C3H2, the singly
deuterated c-C3HD, and the species with one 13C off of the principal axis of
the molecule, c-H13CC2H. The lines of c-C3D2 have been observed with high
signal to noise ratio, better than 7.5 sigma in TMC-1C and 9 sigma in L1544.
The abundance of doubly deuterated cyclopropenylidene with respect to the
normal species is found to be (0.4 - 0.8)% in TMC-1C and (1.2 - 2.1)% in L1544.
The deuteration of this small hydrocarbon ring is analysed with a comprehensive
gas-grain model, the first including doubly deuterated species. The observed
abundances of c-C3D2 can be explained solely by gas-phase processes, supporting
the idea that c-C3H2 is a good indicator of gas-phase deuteration.
