We report the first interstellar detection of c-C3D2. Doubly deuterated cyclopropenylidene, a carbene, has been detected toward the starless cores TMC-1C and L1544 using the IRAM 30 m telescope. The J_{K_a,K_c} = 3_{0,3} {--} 2_{1,2}, 31, 3-20, 2, and 22, 1-11, 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 30, 3-21, 2 and 31, 3-20, 2 lines, belonging to the para and ortho species, respectively. We also observed lines of the main species, c-C3H2, 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 7.5σ in TMC-1C and 9σ 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 analyzed 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. Based on observations carried out with the IRAM 30 m Telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain).
